Document integration of django-payments in a more detailed way

* Once the user picks a payment method, you create a _payment_ object.
* The object in turn provides you a payment form responsible for taking user's input or for submitting necessary information to a third party gateway. The experience depends on a particular payment backend.
* This can result in the payment completing immediately or the user leaving the current page and being redirected to either an external or local page where further interaction may happen (such as PayPal login or 3-D Secure credit card validation).
* Once the process is complete, the payment status changes and a signal is dispatched. The user is finally redirected to either the success or failure URL provided by the payment object.