Improve create payment processor documentation

Created by: artfulrobot

Before

Really hard to write a payment processor; results in wild variations between processors; results in difficult to maintain code and inconsistencies.

After

Include documentation on the responsibilities and roles of the various payment processor classes; what data and logic they should handle and what they should not.

PR WIP

What do you think about this? @eileenmcnaughton @mattwire

I'm desperately trying to do things right while keeping things working on the GoCardless extension, but every example I find to reference seems riddled with inconsistencies, which I suspect is to do with things being written at different points in time; guesswork; bodges/hacks for particular awkward third parties; changes in core (like Order API)... The existing documentation has loads of problems IMO, so rather than just sigh I thought I'd have a stab at setting stuff straight.

But this may not be the best way to do it; I don't know, so don't hold back if you think there's a better way to improve things.