RCBC-179: Update README with the fallback for MacOS X

Change-Id: I3e7b508fced0d3f7c156415ddb055aff429b9a05
Reviewed-on: http://review.couchbase.org/40730
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>