message: ops: document message relayer usage
  The message relayer is turned off by default,
  mention how to turn it on in the docs.
diff: @@ -52,6 +52,17 @@ A Makefile has been provided for convience. The following targets are available.
  - make up-metrics
  - make down-metrics
  +## Cross domain communication
  +
  +By default, the `message-relayer` service is turned off. This means that
  +any tests must manually submit withdrawals. The `message-relayer` will
  +automatically look for withdrawals and submit the proofs. To run with the
  +`message-relayer` on, use the command:
  +
  +```bash
  +$ docker-compose up --scale relayer=1
  +```
  +
  ## Authentication
  Influxdb has authentication disabled.
  