added comment on how to get a default config file

docker-compose.yaml

@@ -5,6 +5,7 @@ services:
     ports:
       - "59661:59661"
     volumes:
+      # generate a sample config.toml by running "docker run diflouroborane --print-default". You may have to delete the ipv6 listen section.
       - "$PWD/config.toml:/etc/diflouroborane.toml"
     links:
       - mqtt