Add generic auth plugin documentation
Add generic auth plugin documentation

Add some documentation regarding the existence of generic plugins. Those
that can be used against the v2 or v3 APIs.

Change-Id: Ie52f0653e20cbc9338481f874aaefa4cdee97116

klmitch/python-keystoneclient,ging/python-keystoneclient,sdpp/python-keystoneclient,magic0704/python-keystoneclient,magic0704/python-keystoneclient,sdpp/python-keystoneclient,Mercador/python-keystoneclient,ging/python-keystoneclient,darren-wang/ksc,Mercador/python-keystoneclient,darren-wang/ksc,klmitch/python-keystoneclient