Update README.rst

Add windows (cygwin) install instructions.

README.rst

@@ -133,6 +133,28 @@ failing, e.g.::
 On OSX, if you encounter clang: error: unknown argument: '-mno-fused-madd' you
 will need to export CFLAGS=-Qunused-arguments.
 
+Windows Install
+---------------
+Synapse can be installed on Cygwin. It requires the following Cygwin packages:
+
+ - gcc
+ - git
+ - libffi-devel
+ - openssl (and openssl-devel, python-openssl)
+ - python
+ - python-setuptools
+
+Troubleshooting:
+
+- You may need to upgrade ``setuptools`` to get this to work correctly:
+  ``pip install setuptools --upgrade``.
+- You may encounter errors indicating that ``ffi.h`` is missing, even with
+  ``libffi-devel`` installed. If you do, copy the ``.h`` files:
+  ``cp /usr/lib/libffi-3.0.13/include/*.h /usr/include``
+- You may need to install libsodium from source in order to install PyNacl. If
+  you do, you may need to create a symlink to ``libsodium.a`` so ``ld`` can find
+  it: ``ln -s /usr/local/lib/libsodium.a /usr/lib/libsodium.a``
+
 Running Your Homeserver
 =======================