Usage documentation

[dbu]

I think it would make sense to insert a chapter Usage into the documentation. Normally i would put it after Configuration but with this library, Configuration is only necessary for advanced use cases. maybe even warn about that (you probably don't need to configure anything) in the intro to configuration.

the phpunit annotation doc could be moved from "Installation" to Usage. and it should explain the concept of a cassette and the main factory calls. also should explain the thing about / in cassette names to structure in subdirectories.