README: indicate what this branch is for

Indicate this is a maintenance branch

Change-Id: I902756f36eaf3dfefdd2e452e39936130890a131
Reviewed-on: http://review.couchbase.org/38695
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>