Added heading level standard

Author: wouterj

contributing/documentation/standards.rst

@@ -7,6 +7,8 @@ look and feel familiar, you should follow these standards.
 Sphinx
 ------
 
+* The following characters are choosen for different heading levels: level 1
+  is ``=``, level 2 ``-``, level 3 ``~``, level 4 ``.`` and level 5 ``"``;
 * Each line should break approximately after the first word that crosses the
   72nd character (so most lines end up being 72-78 characters);
 * The ``::`` shorthand is *preferred* over ``.. code-block:: php`` to begin a PHP