Re: [browser-payment-api] What is the format for payment method identifiers for distributed extensibility (#11)

Migrating from https://github.com/w3c/webpayments/issues/25:

The [Payment Request Architecture diagram](http://wicg.github.io/paymentrequest/specs/architecture.html#architecture) specifies a Payment Method Identifier Registry and suggests that it may be a wiki. Who has write access to that wiki and what is the structure of that wiki?

I suggest that instead of a wiki, we use a document that is controlled by the Web Payments IG. The process for getting something into that document doesn't need to be complex (as simple as a pull request or an email to the group), but ultimately, entries into that document should be discussed.

For example, there are [669 cryptocurrencies](https://en.wikipedia.org/wiki/List_of_cryptocurrencies), do all of them get to update their entries in the registry? Is the registry open to all?

I suggest we use URLs to identify payment types. Then we make the registry necessary ONLY if you want a short-name like "Visa" or "MasterCard", or you want to document how you are compatible with the Web Payments standard (standardized request format and response format).

The other thing we want to ensure is that there is some discussion upon the integration of a new payment method (actual standardization discussion, not just one person coming in and slapping their idea on a page).  To be clear, none of this should prevent anyone from launching their own payment method. The registry is an optimization and information repository and is not required for new participants to enter the ecosystem.

I propose that, at a minimum, the following data is in each entry in the registry:
* payment method URL (the identifier)
* shortcode (like "Visa", "Bitcoin", etc.)
* payment request format
* payment response format

I propose that the best format for this document is as a Linked Data Vocabulary with a corresponding JSON-LD context as that formalizes how the data is expressed and represented across different syntaxes.

Spec refs:
http://web-payments.github.io/web-payments-messaging/#payment-instrument-registration
http://web-payments.github.io/web-payments-messaging/#payment-request
http://web-payments.github.io/web-payments-messaging/#payment-acknowledgement
http://wicg.github.io/web-payments-browser-api/#payment-instrument-registration
http://wicg.github.io/paymentrequest/specs/architecture.html#payment-method-registration-specifications
http://wicg.github.io/paymentrequest/specs/method-identifiers.html#issue-1
http://wicg.github.io/paymentrequest/specs/method-identifiers.html#issue-2


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