Initial device resources doc.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

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+## Device Resources
+
+The device resources schema is intended to be a complete description of an
+island-based FPGA design.  It is made of many components, but the core
+description of the device is shown below.
+
+```
+┌─────────────────┐
+│  Device         │
+│ ┌─────────────┐ │
+│ │ Tile        │ │
+│ │ ┌─────────┐ │ │
+│ │ │ Site    │ │ │
+│ │ │ ┌─────┐ │ │ │
+│ │ │ │ BEL │ │ │ │
+│ │ │ └─────┘ │ │ │
+│ │ │         │ │ │
+│ │ └─────────┘ │ │
+│ │             │ │
+│ └─────────────┘ │
+│                 │
+└─────────────────┘
+```
+
+That is:
+ - A device contains tiles
+ - Tiles contains sites
+ - Sites contains BELs
+
+The schema contains the required information to answer questions such as:
+ - Where are tiles are located?
+ - How are sites connected to the routing graph?
+ - How are BELs connected to the boundry of the site?
+ - How can cells be placed at BELs?
+
+## Terminology
+
+- Device - A set of tiles and package pins.
+- Tiles - An instance of a tile type which contains wires and sites
+- Package pin - A boundry between the "interior" of the device and what is "outside" the package.  Generally corrisponds to a pin on a package, e.g. pin 1 on SOP-8 or A1 on CSG324.
+- Wire - Also known as a "tile wire" .  A wire is a piece of conductive material totally contained within a tile.  Wires can be part of nodes.  Wires can connect to PIPs or site pins.
+- Node - A node is a set of 1 or more wires that are connected.  Nodes can span multiple tiles.  Nodes connect to PIPs or site pins via the wires that are part of the node.
+- PIP - PIP is an abreviation for programable interconnect point.  A PIP provides a connection between two wires.  PIPs can be unidirectional or bidirectional.  Unidirectional PIPs always connect wire0 to wire1.  Bidirectional PIPs can connect wire0 to wire1 or wire1 to wire0.
+- Site - A collection of site pins, site wires and BELs.
+- Site pin - The connect between a site and a wire. Site pins may connect to 0 or more site port BELs.
+- Site wire - A piece of conductive material that connects to at most 1 output BEL pin and 0 or more input or inout BEL pins.
+- BEL - BEL is an abreviation of basic logic element.  A BEL can be one of 3 types, site port, logic, routing.  A BEL contains 1 or more BEL pins.
+- BEL pin - A connection between a BEL and a site wire.
+- Logic BEL - A placable logic element.  May be subject to 0 or more placement constraints.
+- Site port BEL - A site port BEL represents a connection to a site pin contained within the parent tile of the site.  See [Site port BEL](#site_port_bel).
+- Routing BEL - A routing BEL connects at most 1 input BEL pin to the output BEL pin.  See [Routing BEL](#routing_bel).
+- Site PIP - A pair of input and output BEL pins belonging to a BEL that represents a logically connection.
+- Cell - A logical element of a design that some number of cell ports and some number of cell instances, and some number of nets.
+- Cell port - The boundry between the interior of a cell and the containing cell (if any).
+- Cell instance - A instance of a cell.  The cell ports of may be connected to nets within the parent cell.
+- Net - A set of logically connected cell ports.
+
+## The place and route problem
+
+The device resources schema is intended to provide a description for a tool
+solving the place and route problem. The definition of the problem used by
+this schema is described below:
+
+There exists exactly 1 **cell instance** (the **top** instance) that contains 1 or
+more leaf **cell instances** that must be placed at **BELs**, such that no
+**constraints** are violated and the **nets** between the **cell instances** are
+**routable**. **Routable** means that **site wires**, **site PIPs**,  **nodes**, **PIPs**
+can be assigned to at most 1 **net** such that each **net driver BEL pin** can
+reach each every **net sink BEL pin** on the **net**.
+
+The device resource format describes how **cell instances** can be legally
+placed at **BELs** and how **cell pins** relate to **BEL pins**.  When a
+**cell instance** is placed at a **BEL**, it may be subject to 0 or more
+**constraints**.
+
+**Nets** are divided into 3 categories.  A **signal** net represents a signal
+that is not either the constant logical 0 or constant logical 1 net.
+The constant logical 0 and constant logical 1 nets are special because they
+can have multiple drivers in the device description.  Routing resources that
+are always part of the constant logical 0 or constant logical 1 net are
+explicitly defined in the device resources scheam.  The constant logical 0 net
+is listed in the schema as the "gnd" type.  The constant logical 1 net is
+listed in the schema as the "vcc" type.
+
+### Rules for routing
+
+Fully routed signal nets always begin at a output/inout BEL pin, and always
+end at an input/inout BEL pin.  If a net enters a site, that net **must** end
+at an input/inout BEL pin.  It is not legal for a net to enter and leave a
+site.  If such a path is required, a pseudo PIP should be added to the schema.
+
+## Details
+
+### Net routing summary
+
+Each net start at the driver output/inout BEL pin.  The BEL pin will be
+connected to exactly 1 site wire.  If the net sink can be reached within the
+site, then the net can use site PIPs (through a routing BEL) to reach a site
+wire connected to the input/inout BEL pin.
+
+If the net sink is in another site, then the net must first reach a site port
+input BEL pin using site PIPs to reach the site wire connected to the site
+port. From there the net leaves the site via the site port and is now on the
+first node via the site pin matching the site port.
+
+From there the net must use PIPs to expand to new nodes until arriving at a
+node attached to valid site pin for the sink.  This would be a site pin that
+is part of the same site that the sink BEL is part of, and that the site port
+wire can reach the sink BEL pin (via 0 or more site PIPs). The site can be
+entered via the site port corrisponding to the site pin. The first site wire
+in the site will be the site wire attached to the output BEL pin of the site
+port.  From there site routing continues per above.
+
+![Wire and nodes](https://symbiflow.readthedocs.io/projects/arch-defs/en/latest/_images/rrgraph-wire.svg)
+
+### Tile Types and site types
+
+To reduce data duplication in the device schema, both tiles and sites have a
+type.  Most of the definition of the tile and site is in the type rather than
+repeated at each instance.  This does cause some more complicated
+indirection, so the following section provides some additional explaination
+here.
+
+#### Sites, site types and alternative site types
+
+The most complicated relationship in the schema is likely the relationship
+between sites, site types and alternative site types.
+
+Most of the site type description is independent of the tile / tile type that
+the site type is within.  See the "SiteType" struct definition for
+the independent portion of the schema. The important exception is the
+relationship between wires and site pins.
+
+Each site within a tile has a "primary site type", which is found in the
+"SiteTypeInTileType" struct definition, contained in the "TileType" struct.
+The site within the tile will specify which "SiteTypeInTileType" to use for
+that particular site.
+
+The primary site type contains a list of "alternative" site types that may be
+used ("altSiteTypes" in "SiteType").  The primary site type must always
+contains the complete list of site pins used in any of the alternative site
+types.
+
+The site pins to wire relationship is always done via the primary site type.
+When an alternative site type is used, the site pins of that alternative site
+type must be first be mapped to a site pin of the primary site type.
+
+The "SiteTypeInTileType" defines the relationship between the primary site
+type and the wires.  It also defines the relationship from the alternative
+site type to the primary site type.
+
+It first defines the primary site type ("primaryType").  It defines a map
+between the site pins in the primary site type and wires in the tile type that
+contains the site ("primaryPinsToTileWires").  Last it defines
+the map between the alternative site pin and the primary site pin
+("altPinsToPrimaryPins").
+
+Important: When solving the place and route problem, only the primary or one
+of the alternative site types can be used at a time for a particular site.
+
+### <a name="routing_bel">Routing BEL
+
+A routing BEL represents statically configurable site routing by connecting a
+site wire connected to one of the input BEL pins to the output BEL pin of
+the BEL.  Routing BELs may only have 1 output BEL pin.
+
+#### Inverting routing BELs
+
+Some routing BELs can invert signals that pass through them.  Defined the
+"inverting" field in the "BEL" struct with the BEL pins that invert and do
+not invert.
+
+### <a name="site_port_bel">Site port BEL
+
+A site port BEL represents a connection to a site pin contained within the
+parent tile of the site. Site port BELs have exactly 1 BEL pin.  The BEL name
+and pin name should be the same.  The name of the BEL should match the name of
+the site pin that the site port connects too.  The direction of the BEL pin
+should be the opposite of the site pin direction.
+
+Examples:
+
+An input site pin "I0" would have a site port BEL named "I0" with 1 BEL
+output pin named "I0".
+
+## Additional topics
+
+The device resources schema also covers some important data required for
+handling common cases found in island based FPGAs.
+
+### Signal inversions
+
+It is fairly common for fabrics to contain site local signal inverters.
+Depending on the architecture, the place and route tool may be expected to
+leverage inverters or may even require it.  The device resources schemas
+provides a description for how cells express inversion and how to use site
+local inverters to implement the requested inversion.
+
+### LUT definitions
+
+LUTs are common to every island-based FPGA, and many place and route tasks
+depend on having knowledge of how the LUTs are arranged.  The LUT definition
+section of the device resources defines where LUTs exist as BELs and what
+cells can be placed at those BELs.  This is important is at least two place
+and route tasks.  The first is that LUTs can be trivially turned into site pips
+from the input of the LUT to the output of the LUT, subject to **constraints**
+and LUT equation sharing.  The second is that LUTs can be trivially turned
+into constant sources from the output pin.
+
+### Parameters
+
+Some parameters attached to cell instances may be relevant for the place and
+route problem.  A common example is LUT equation sharing, which can happen on
+fracturable LUTs.  See the schema for details.
+
+### Pseudo PIPs
+
+It may be important within a device to represent PIPs that "route-thru"
+one or more BELs.  This can be modelled as placing a cell in a particular
+configuration at a BEL, subject to the normal cell placement rules.  The
+"PseudoCell" struct defines what resources are used by using PIPs.
+
+All pseudo PIPs must define at least 1 pseudo cell.  Pseudo cells should
+include the site port BEL that the pseudo PIP used to enter the site.