Add example for a multicluster client

Change-Id: Ie3924550c07ed92096c5a90b7aa0a595daa6cd65
Reviewed-on: http://review.couchbase.org/28221
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>