Fix the recommended Sphinx version.

The latest Sphinx 1.4.4 produces lots of warnings about four-column indices.
We'll wait for Read the Docs to upgrade their systems before moving to the newer
Sphinx version.

README.rst

@@ -42,8 +42,11 @@ Building the documentation
 To build the Cretonne documentation, you need the `Sphinx documentation
 generator <http://www.sphinx-doc.org/>`_::
 
-    $ pip install sphinx
+    $ pip install sphinx==1.3.5 sphinx-autobuild
     $ cd cretonne/docs
     $ make html
     $ open _build/html/index.html
 
+The specific Sphinx version is currently used by Read the Docs. Sphinx 1.4 has
+been released, but produces lots of warnings about four-column indices. We'll
+upgrade when Read the Docs does.