Added a note about writing documentation

doc/contributing.rst

@@ -12,6 +12,8 @@ steps.
 
 * Add tests for it. This is important so we don't break it in a future version unintentionally.
 
+* Optionally, add some technical documentation.
+
 * Send a pull request. Bonus points for topic branches.
 
 If you have a big change or would like to discuss something,
@@ -22,3 +24,15 @@ please join us on the `mailing list
 
     Any code you contribute must be licensed under the MIT
     License.
+
+Writing Documentation
+=====================
+
+The documentation is written in `reStructuredText
+<http://docutils.sourceforge.net/rst.html>`_ and can be generated using `sphinx
+<http://sphinx-doc.org>`_.
+
+.. code-block:: bash
+
+    $ cd doc
+    $ sphinx-build -b html . build