PCBC-174: Add example file

This commit documents the CouchbaseClusterManager class in the
example file.

Change-Id: I3e5ac23850fe27ffc582244edc3fcdafff9907be
Reviewed-on: http://review.couchbase.org/23637
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>