Add license file.

Change-Id: I607124ebd030a76ee5a94acbf25772b8688f270a
Reviewed-on: http://review.couchbase.org/42804
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: John Liang <johliang00@gmail.com>