MB-7250:  2.2_v3 README for macosx

Change-Id: I59ccea34ae12910ab14a5352c246f0316814a8f2
Reviewed-on: http://review.couchbase.org/28807
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>