[webpayments] Document relationship diagrams? (#24)

I like the [document diagram](http://wicg.github.io/paymentrequest/specs/architecture.html#architecture) in the Payment Request Architecture document. This diagram should probably go in the capabilities document as the capabilities document is supposed to tie everything together across IGs/WGs.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webpayments/issues/24