Add m.server_notice to reserved tags

specification/modules/tags.rst

@@ -60,6 +60,7 @@ The following tags are defined in the ``m.*`` namespace:
 
 * ``m.favourite``: The user's favourite rooms. These should be shown with higher precedence than other rooms.
 * ``m.lowpriority``: These should be shown with lower precedence than others.
+* ``m.server_notice``: Used to identify `Server Notice Rooms <#module-server-notices>`_.
 
 {{m_tag_event}}