Adding note about contributing through Gerrit

Change-Id: Ia0b97ca57d80513414a7e3e7ca4cad3eae4dbdca
Reviewed-on: http://review.couchbase.org/42645
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>