[MB-10281]: Update readme file for 2.5.1 for MAC OS.

Change-Id: Iecd693d91cb06dc865c574fa1dd651fa104f7f25
Reviewed-on: http://review.couchbase.org/34406
Reviewed-by: Eric Limare <eric.limare@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>