doc: add readdir-ahead documentation

Change-Id: Ib6fc3c9b59b498bfe3fb3dfc23dca874bdc0d21d
BUG: 1086755
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/7904
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>