Add link to user-agent documentation
So the user has a pointer to the documentation about the searched topic.
@@ -21,7 +21,8 @@ Custom User-Agents?
-------------------
Requests allows you to easily override User-Agent strings, along with
-any other HTTP Header.
+any other HTTP Header. See `documentation about headers <http://docs.python-requests.org/en/master/user/quickstart/#custom-headers>`_.
+
Why not Httplib2?
