Merge pull request #525 from skeuomorf/installation-docs

Added canonical installation document with details about various platforms

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+Installing
+==========
+
+You can install ``cryptography`` with ``pip``:
+
+.. code-block:: console
+
+    $ pip install cryptography
+
+Installation Notes
+==================
+On Windows
+----------
+If you're on Windows you'll need to make sure you have OpenSSL installed.
+There are `pre-compiled binaries`_ available. If your installation is in
+an unusual location set the ``LIB`` and ``INCLUDE`` environment variables
+to include the corresponding locations. For example:
+
+.. code-block:: console
+
+    C:\> \path\to\vcvarsall.bat x86_amd64
+    C:\> set LIB=C:\OpenSSL-1.0.1f-64bit\lib;%LIB%
+    C:\> set INCLUDE=C:\OpenSSL-1.0.1f-64bit\include;%INCLUDE%
+    C:\> pip install cryptography
+
+Using your own OpenSSL on Linux
+-------------------------------
+
+Python links to OpenSSL for its own purposes and this can sometimes cause
+problems when you wish to use a different version of OpenSSL with cryptography.
+If you want to use cryptography with your own build of OpenSSL you will need to
+make sure that the build is configured correctly so that your version of
+OpenSSL doesn't conflict with Python's.
+
+The options you need to add allow the linker to identify every symbol correctly
+even when multiple versions of the library are linked into the same program. If
+you are using your distribution's source packages these will probably be
+patched in for you already, otherwise you'll need to use options something like
+this when configuring OpenSSL:
+
+.. code-block:: console
+
+    $ ./config -Wl,--version-script=openssl.ld -Wl,-Bsymbolic-functions -fPIC shared
+
+You'll also need to generate your own ``openssl.ld`` file. For example::
+
+    OPENSSL_1.0.1F_CUSTOM {
+        global:
+            *;
+    };
+
+You should replace the version string on the first line as appropriate for your
+build.
+
+Using your own OpenSSL on OS X
+------------------------------
+
+To link cryptography against a custom version of OpenSSL you'll need to set
+``ARCHFLAGS``, ``LDFLAGS``, and ``CFLAGS``. OpenSSL can be installed via
+`Homebrew`_:
+
+.. code-block:: console
+
+    $ brew install openssl
+
+Then install cryptography linking against the brewed version:
+
+.. code-block:: console
+
+    $ env ARCHFLAGS="-arch x86_64" LDFLAGS="-L/usr/local/opt/openssl/lib" CFLAGS="-I/usr/local/opt/openssl/include" pip install cryptography
+
+
+.. _`Homebrew`: http://brew.sh
+.. _`pre-compiled binaries`: https://www.openssl.org/related/binaries.html