New PaymentRequest Doc

Hi all -

I wanted to let everyone know that we just published a new rationale
document to the PaymentRequest repo titled, "Why PaymentRequest". You can
read it here:
https://github.com/WICG/paymentrequest/blob/gh-pages/docs/rationale.md

The goal of this doc is to clarify in a single place our goals with the
PaymentRequest API. We've talked about these at various points over the
last few months (on phone calls, in person, etc), but I thought it would be
valuable to have them actually written down somewhere for reference and
easy access.

The primary audience for this document are either new members to the WG or
guests that are curious about the proposed API and what key problems we're
trying to solve.

Feedback always welcome. :)

Thanks,

Zach