Seminar demo: document how to make multiple hosts

[edemaine]

I was trying the following workflow:

• In one browser, open https://rajgoel.github.io/reveal.js-demos/seminar-demo.html and click "host seminar".
• In another browser, open https://rajgoel.github.io/reveal.js-demos/seminar-demo.html and click "host seminar".
• I expected these to be in the same room, but they end up with different random room names.

It might be worth adding some documentation to the demo to explain this feature. For example, on the "host seminar" slide, we could mention something like:

• If there isn't a room name in the URL (...?room=ROOM_NAME), it will generate one randomly.
• If you're already in a room, you will become an (additional) host instead of a viewer of that room.

Originally posted by @edemaine in #121 (comment)

[rajgoel]

Thanks, the documentation certainly needs to be improved and showcases of custom setups should be provided.