Adding admin-guide for distributed-geo-rep

Change-Id: Icbbc9d3fc14fc1a5ae9550f096eb69dda41e92a4
BUG: 1086747
Signed-off-by: M S Vishwanath Bhat <vbhat@redhat.com>
Reviewed-on: http://review.gluster.org/7438
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>