Document extensions to jquery.payment

[cancan101]

Add to readme notes on what is added by this issuance vs jquery.payment.

[jondavidjohn]

While this is a derived work from jquery.payment, I am not proposing it is a drop in replacement and/or should maintain compatibility, it is possible (though not likely) they could diverge significantly. Wouldn't want to add yet another collection of documentation notes to maintain.

Thanks for the idea though!