This updates our existing sample docker compose config files to show how to

launch Review Board alongside an instance of Doc Converter, the document

conversion service needed for Power Pack's document review feature.
The configs now have an instruction step for dealing with "optional services",

which are any services that we may include in the file but are optional. For

now RabbitMQ and Doc Converter are the only optional services, but in the

future we'll add Review Bot too.
The instructions for the RabbitMQ service make it clear that it can be

used with multiple services, not just Doc Converter, and lists the

services that require it.

Tested this sample docker-compose and successfully ran the containers. Was

able to upload documents to the Review Board server and have them converted.