doc: add documentation for NUFA feature

Change-Id: I00f1c669d4ebc61f09c50f8de4c893907f6d75c2
BUG: 1086745
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/7489
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>