Add link to Sphinx installation

Author: ddeboer

doc/contributing.rst

@@ -46,21 +46,24 @@ To run the functional tests:
 
 For more information about testing, see :doc:`/testing-your-application`.
 
-.. _HHVM: http://www.hhvm.com/
-.. _HHVM FastCGI server: https://github.com/facebook/hhvm/wiki/fastcgi
-
 Building the Documentation
 --------------------------
 
-First download the requirements:
+First `install Sphinx`_, then download the requirements: 
 
 .. code-block:: bash
 
     $ pip install -r doc/requirements.txt
     
-Then you can build the docs:
+To build the docs:
 
 .. code-block:: bash
 
     $ cd doc
     $ make html
+
+.. _HHVM: http://www.hhvm.com/
+.. _HHVM FastCGI server: https://github.com/facebook/hhvm/wiki/fastcgi
+.. _install Sphinx: http://sphinx-doc.org/latest/install.html
+
+