added README for saslauthd-port

Change-Id: Ibaf073149ec3d78f3265b4e23c236b420942c14a
Reviewed-on: http://review.couchbase.org/44116
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>