docker-compose: add example for `--monitor-system-unit`

amendment to https://github.com/fphammerle/systemctl-mqtt/commit/6217841d478cf52dcf9d455f92e3681bfdc8d9b3

docker-compose.yml

@@ -31,6 +31,7 @@ services:
       --mqtt-disable-tls
     #  --mqtt-username raspberrypi
     #  --mqtt-password-file /etc/systemctl-mqtt/mqtt-password
+    #  --monitor-system-unit ssh.service
     hostname: raspberrypi
     userns_mode: host
     # prefering explicit user specification over "USER 0" in Dockerfile