Adding doc to upgrade geo-rep from older version to new geo-rep

Change-Id: Ibfebca2bf92e63b906c9eb336b4a975c534a0bf2
BUG: 1086796
Signed-off-by: M S Vishwanath Bhat <vbhat@redhat.com>
Reviewed-on: http://review.gluster.org/7436
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>