Add basic documentation to repo

[WilliamRoyNelson]

At a minimum, there should be a documented "help" or "usage" command, that allows for further info from the bot directly.

Ideally, each command would be documented online.

[monodop]

Yep, I've been putting this off but we do need a good user guide.

• starting and running the docker image
• environment variables you can tweak
• available commands for chat & voice
• building and running the bot from source code