Add documentation on adding a new payment processor

[nileshtrivedi]

I'd like to add support for additional payment gateways but couldn't find any relevant documentation anywhere. Here's the implementation of Stripe which is not very well commented: https://github.com/liberapay/liberapay.com/blob/master/liberapay/payin/stripe.py

[Changaco]

The only developer documentation we have is the README. If you want to contribute, then you'll have to figure out the rest for yourself, and ask me if you get stuck.

I'm not aware of any good payment processor that we could add support for. The existing issue for this is #1394.