Provide concrete example of prefetch usage

I recently suffered through trying to get Queue prefetch working. Neither this gem's nor Rabbit's own docs explained how all of the pieces should go together.  

The existing gem docs mention the `Channel#prefetch` method which sends the basic.qos packet.  However, the docs did not explain the `Queue#subscribe` `ack: new` argument nor the `AMQP::Header#ack` method necessary to connect the dots.

This PR adds a trivial example usage putting all of the pieces together.

Thanks!