transport/rdma: Add rdma connection manager documentation

Change-Id: I15ad9cafbe607074ddb9b36f9e879c8421f6eae2
BUG: 1086743
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/7900
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>