Document API changes and migration path

[chillu]

Given the module had a major overhaul, we don't need deprecation cycles or exhaustive docs. Upgrading from earlier use of the payment module is expected to be non-trivial.

Still, it would be good to give an overview on what devs can expect when upgrading. Example:

• Moved DPS/PaymentExpress into new "payment-paymentexpress" module ()
• Moved "test harness" into new "payment-test" module ()
• Moved payment processing from Payment->processPayment() into a new PaymentProcessor API (<example with before/after code>)
• ...