[TASK] Add a legend for colors associated to LDAP groups

Resolves: #60973
Change-Id: I499482643c8ad52d8ec448e7be60ee2adafee9c3
Reviewed-on: http://review.typo3.org/32164
Reviewed-by: Thierry Brodard <thierry.brodard@techniconcept.ch>
Tested-by: Thierry Brodard <thierry.brodard@techniconcept.ch>