Document `playhouse` model paths
Awesome, thanks!

I was working through the peewee documentation on [PickledField](http://docs.peewee-orm.com/en/latest/peewee/playhouse.html#PickledField). However, I could find nowhere in the documentation where to import this from.

I tried:
- the documentation page, near as I can tell, doesn't list this directly
- looking at the page documentation source, but there's no `current module` directive
- `PickledField` is listed in neither `dir(peewee)` or `dir(playhouse)`

This seems to be the case for all the classes listed under Playhouse.

I would recommend listing the import path somewhere in the documentation. Maybe just under each class:

> available at `playhouse.fields.PickledField`
