Add Windows AD documentation

Added documentation to setup swiftkerbauth environment
with Windows Active Directory Server and Linux client

Change-Id: I18333428c633b23fd15afc8965266d546f0bb03b
Original-author: Chetan Risbud <crisbud@redhat.com>
Signed-off-by: Prashanth Pai <ppai@redhat.com>
Reviewed-on: http://review.gluster.org/6041
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>