Revise readme to help anyone trying to learn CE

[Legioth]

Now when the code for this demo is made publicly available, it might make sense to add some helpful details to the readme file.

Some things that come to mind

• Warn about the special solutions that we have in the demo just to make it work better as a demo but isn't representative of typical application code: the bots and the special way of "logging in".
• Reference to relevant overall documentation
• Mention the specifics of running this application, i.e. logging in to vaadin.com to get a "subscription" and then the special considerations for production use (maybe just a link to specific parts of documentation)