Link to [[mw:Help:Extension:MassMessage]] at top of the special page

Change-Id: I0a21d3dc412004626bcb34a85a2def3412a87567
Suggestion: https://www.mediawiki.org/wiki/Thread:Extension_talk:MassMessage/Add_a_link_to_the_help_page_from_within
Merge "Link to [[mw:Help:Extension:MassMessage]] at top of the special page"