Rename CONTRIBUTING to CONTRIBUTING.rst for Gitlab formatting preview.

Rename occurences of the file through the documentation.

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

README.rst

@@ -26,7 +26,7 @@ Contribute
 ----------
 
 Contribution guidelines as well as development installation instructions
-are outlined in `CONTRIBUTING <CONTRIBUTING>`_.
+are outlined in `CONTRIBUTING <CONTRIBUTING.rst>`_.
 
 Translate
 ^^^^^^^^^

docs/contributing.rst

-.. include:: ../CONTRIBUTING
+.. include:: ../CONTRIBUTING.rst