Add documentation for the Facebook backend

[scarlehoff]

The facebook backend seems to work well (although basically a beta right now since I'll have to test it a lot to be sure) but it needs documentation, because setting everything up is very much not trivial.

• Add some kind of auto_config for getting the chat id
• Add in the readme a configuration guide to see how to create an new app with facebook (which is less than trivial). Also explain that one needs a SSL-ready server and so on.
• Add a webhook script for doing the verification of the token, which is also less than trivial.