Add documentation to set this up locally

Add full instructions how to set everything up (including match maker) to get this running at a custom domain.

This wasn't done earlier, because I didn't feel any urge to do this and I didn't know how popular this eventually be (I have no tracking or any other data aggregation build in to get any numbers).

To do this, we need some steps to be done first.

1. Add usage policy. I want feedback and I don't want my name to be removed.
2. Check if any domains are hard coded. If so, make them optional. This should be essentially a NOOP, because I have this running locally. I think its better to do this anyway.
3. Think about how to relay Github updates to third party installations. First party is just done using webhooks but how about third party?
4. Document how to setup pronto.
5. Document how to setup this project. This is mostly `docker-compose up` but we have some variables to fill in.
6. Document how to update (or auto update) this project correctly.
7. Document how to list own installations to provide a larger community.

The most critical part is task 3. I don't want any old installations lingering around and creating a bad reputation of my project. The main installation is just done using web hooks and the server rebuilds everything in the background and starts the lobby but this cant work for any installation. I think the best for such cases would be, to have the server pull updates periodically and show a banner if such updates exists. Plain auto updates won't work, because we can't rebuild the docker container inside the docker container itself (I know of a workaround but I won't consider it for security reasons).