add a paragraph about sphinx-contrib on the front page

doc/_templates/index.html

@@ -34,6 +34,9 @@
     <li>{%trans path=pathto('extensions')%}<b>Extensions:</b> automatic testing of code snippets, inclusion of
       docstrings from Python modules (API docs), and
       <a href="{{ path }}#builtin-sphinx-extensions">more</a>{%endtrans%}</li>
+    <li>{%trans path=pathto('develop')%}<b>Contributed extensions:</b> more than
+      50 extensions <a href="{{ path }}#extensions">contributed by users</a>
+      in a second repository; most of them installable from PyPI{%endtrans%}</li>
   </ul>
   <p>{%trans%}
     Sphinx uses <a href="http://docutils.sf.net/rst.html">reStructuredText</a>