Add docs for "magic" methods

[garrypolley]

Patio adds a lot of power via its model layer and the associations.

Currently I do not think there are any docs for the automatically added methods.

The

• model.addRelated
• model.removeRelated

As well as other magic methods are not documented. It'd be great to add it to the documentation so people browsing the docs knew they were there. They are tested, which is awesome, so it should be a fairly easy add.