Add documentation around overriding `initWithDictionary:error:`

Getting started with Mantle, I ran into the same confusion as #336 and wrote up some notes.
> How about some form UUID?

Hmm, well, that's the kind of thing I'd actually put into `-init`.

What about validating the existence of a particular property? That can't be done with `-validateValue:forKey:error:`, since it's not called for absent keys.