Added example how to integrate 'cash-on-delivery'

There's no extension for it at the moment, but it seems to confuse
users. See #995.

This reflects the change in the repo URL.

During many payment flows, the user is able to cancel the process.
This should often be treated differently from a payment error,
e.g. it might not be appropriate to offer to retry the payment.

This exception adds gives us a good way to address that case.

Closes #1528.

Fixes #1145

Fixes #1028