Document how `xontrib` modules are registered and loaded through `setuptools`

I think it would be super helpful to have some comments inside your example `setup.py` script which (briefly) explain how `xontrib` module registration works in terms of what `xonsh` will do to load such registered modules.

So for example [here you could document what and (ideally how) modules under `xontrib/` are processed and made available in some env to `xontrib load `](https://github.com/anki-code/xontrib-rc-awesome/blob/495dce4c8e7e8c9882ea002db60935d03f3fb861/setup.py#L39)