Update README.md with shield, pros/cons table...

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+[![Documentation Status](https://readthedocs.org/projects/ghdl/badge/?version=latest)](http://ghdl.readthedocs.io/en/latest/?badge=latest) [![Join the chat at https://gitter.im/ghdl1/Lobby](https://badges.gitter.im/ghdl1/Lobby.svg)](https://gitter.im/ghdl1/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![GNU General Public License 2](https://img.shields.io/github/license/tgingold/ghdl.svg?style=flat)](https://github.com/tgingold/ghdl/blob/master/COPYING)
+
+[![Linux containers at Travis-CI](https://travis-ci.org/tgingold/ghdl.svg?branch=master)](https://travis-ci.org/tgingold/ghdl) [![Windows VMs at AppVeyor](https://ci.appveyor.com/api/projects/status/rsq60m5wcly3og8j?svg=true)](https://ci.appveyor.com/project/tgingold/ghdl) ![Latest tag](https://img.shields.io/github/tag/tgingold/ghdl.svg?style=flat) [![Latest release](https://img.shields.io/github/release/tgingold/ghdl.svg?style=flat)](https://github.com/tgingold/ghdl/releases)
+
 # GHDL
 
-[![Join the chat at https://gitter.im/ghdl1/Lobby](https://badges.gitter.im/ghdl1/Lobby.svg)](https://gitter.im/ghdl1/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) 
+This directory contains the sources of GHDL, the open-source compilator and simulator for [VHDL](https://en.wikipedia.org/wiki/VHDL), a Hardware Description Language ([HDL](https://en.wikipedia.org/wiki/Hardware_description_language)). GHDL is not an interpreter: it allows you to analyse and elaborate sources to generate machine code from your design. Native program execution is the only way for high speed simulation.
 
-Build status:
-	- [![Linux containers at Travis-CI](https://travis-ci.org/tgingold/ghdl.svg?branch=master)](https://travis-ci.org/tgingold/ghdl)
-	- [![Windows VMs at AppVeyor](https://ci.appveyor.com/api/projects/status/r5dtv6amsppigpsp/branch/release?svg=true)](https://ci.appveyor.com/project/Paebbels/poc/branch/release)
-Distribution:
-	- ![Latest tag](https://img.shields.io/github/tag/tgingold/ghdl.svg?style=flat)
-	- [![Latest release](https://img.shields.io/github/release/tgingold/ghdl.svg?style=flat)](https://github.com/tgingold/ghdl/releases)
-	- ![GNU General Public License 2](https://img.shields.io/github/license/tgingold/ghdl.svg?style=flat)
-	
----
-	
-This directory contains the sources of GHDL, the VHDL simulator.
+## Main features
+
+Full support for the [1987](http://ieeexplore.ieee.org/document/26487/), [1993](http://ieeexplore.ieee.org/document/392561/), [2002](http://ieeexplore.ieee.org/document/1003477/) versions of the [IEEE](www.ieee.org) [1076](http://standards.ieee.org/develop/wg/P1076.html) VHDL standard, and partial for the latest [2008](http://ieeexplore.ieee.org/document/4772740/) revision.
+
+Partial support of [PSL](https://en.wikipedia.org/wiki/Property_Specification_Language).
+
+By using a code generator ([LLVM](http://llvm.org/), [GCC](http://gcc.gnu.org/) or, [x86_64](https://en.wikipedia.org/wiki/X86-64)/[i386](https://en.wikipedia.org/wiki/Intel_80386) only, a built-in one), it is much faster than any interpreted simulator. It can handle very large designs, such as [leon3/grlib](http://www.gaisler.com/index.php/downloads/leongrlib).
+
+GHDL runs on [GNU/Linux](http://en.wikipedia.org/wiki/Linux_distribution), [Windows](http://en.wikipedia.org/wiki/Microsoft_Windows) and [macOS](http://en.wikipedia.org/wiki/MacOS), both on `x86` and on `x86_64`. You can freely download a binary distribution for your OS, or try to build it on your own machine (see *'Getting GHDL'* below).
+
+Can write waveforms to a [GHW](http://ghdl.readthedocs.io/en/latest/using/Simulation.html?highlight=GHW#cmdoption-wave), [VCD](https://en.wikipedia.org/wiki/Value_change_dump) or FST file. Combined with a [GUI](http://en.wikipedia.org/wiki/Graphical_user_interface)-based [waveform viewer](https://en.wikipedia.org/wiki/Waveform_viewer) and a good text editor, GHDL is a very powerful tool for writing, testing and simulating your code.
 
-GHDL is not an interpreter: it generates machine code from your design,
-the only way for high speed simulation.
+Supported third party projects: [VUnit](https://vunit.github.io), [OSVVM](http://osvvm.org), [cocotb](https://github.com/potentialventures/cocotb) (through the [VPI interface](https://en.wikipedia.org/wiki/Verilog_Procedural_Interface)), ...
 
-GHDL fully supports IEEE 1076-1987, IEEE 1076-1993, IEEE 1076-2002 and
-partially the 1076-2008 version of VHDL.
+GHDL is free software:
 
-Main features are:
-- available on Linux (x86 and x86-64), Windows and Mac OS x
-- handle very large designs like leon3/grlib.
-- can write waveforms to a vcd or fst file.
-- partial support of PSL
-- support vunit (https://vunit.github.io)
-- support OSVVM (http://osvvm.org)
-- support cocotb (https://github.com/potentialventures/cocotb) through the VPI interface
+- [![GNU General Public License 2](https://img.shields.io/github/license/tgingold/ghdl.svg?style=flat)](https://github.com/tgingold/ghdl/blob/master/COPYING)
+- Doc is available on [ghdl.readthedocs.org](https://ghdl.readthedocs.org/en/latest/index.html).
+- Some of the runtime libraries, are under different terms; see the individual source files for details.
 
-GHDL is free software.  See the file COPYING for copying permission.
-The manuals, and some of the runtime libraries, are under different
-terms; see the individual source files for details.
+## Getting GHDL
 
-Doc is available on https://ghdl.readthedocs.org/en/latest/index.html
+### Pre-built releases
 
-## Building GHDL (Short instructions)
+You can download pre-built releases for different platforms, backends and library versions from [GitHub releases](https://github.com/1138-4EB/ghdl/releases).
 
-If you aren't on linux, see below for full instructions.
+### Building GHDL
 
-You need GNAT GPL 2014 (or later) for x86 (32 or 64 bits).  GNAT is the GNU Ada
-compiler and GNAT GPL is very easy to install (download anonymously from
-libre.adacore.com, untar and run the doinstall script).  You also need
-zlib (for Debian or Ubuntu: install zlib1g-dev package).
+You need the GNU Ada compiler, GNAT GPL, 2014 (or later) for x86 (32 or 64 bits). GNAT GPL can be downloaded anonymously from libre.adacore.com. Then, untar and run the doinstall script.
+
+*Depending on the OS and distribution you are using, you will also need to install some toolchain dependencies, such as `zlib`. See '[Building](http://ghdl.readthedocs.io/en/latest/building/index.html)' for specific package names.*
 
 In the GHDL base directory, configure and build:
 ```sh
@@ -50,14 +44,28 @@ $ ./configure --prefix=/usr/local
 $ make
 ```
 
-At that place, you can already use the 'ghdl_mcode' built in the directory.
-You can also install GHDL (the executable is installed as 'ghdl'):
+At that place, you can already use the 'ghdl_mcode' built in the directory. You can also install GHDL:
+
 ```sh
 $ make install
 ```
 
 That's all!
 
-## Build GHDL (Long instructions)
+*The executable is installed as 'ghdl' in `/usr/local`. To install it to a different path, change the `--prefix` in the call to `configure`. For example, on Windows, you may want to set it to `--prefix=/c/Program Files (x86)/GHDL`.*
+
+---
+
+Furthermore, each supported compiler has its pros and cons. Here is a short comparaison:
+
+| | pros | cons | observations |
+|---|---|---|---|
+|mcode | very easy to build | x86_64/i386 only | no executable created from your design |
+| | very quick analysis time and can analyze very big designs | simulation is slower | |
+| GCC | generated code is faster | analyze can take time (particularly for big units) | the output is an executable |
+| | generated code can be debugged | build is more complex |
+| | many platforms (x86, x86_64, powerpc, sparc) | |
+
+LLVM has the same pros/cons as GCC, but it is easier to build. However, coverage (`gcov`) is unique to GCC.
 
-See [BUILD.txt](https://github.com/tgingold/ghdl/blob/master/BUILD.txt)
+You can find specific instructions for each of the options in '[Building](http://ghdl.readthedocs.io/en/latest/building/index.html)'.