[browser-payment-api] Add section on Extensibility (#44)

Migrating from https://github.com/w3c/webpayments/issues/27:

The Web Payments WG resolved to specify how extensibility of messages is handled in the API specifications to something that looks like this:

"""
In general, the WebIDL descriptions provided in this specification outline the specific interfaces, properties, and values that an implementation may depend on. It is expected that other properties and values will be stored in objects that implement the various interfaces in this specification (e.g. PaymentApplication, PaymentRequest, PaymentAcknowledgement, etc.). While this specification does not suggest a single extension mechanism, it does anticipate extensibility. To that end, Implementations MUST preserve unrecognized properties and their associated values.
Another specification [LINK_TBD] explains how to extend the parameters used with this API using JSON-LD.

ISSUE XYZ:  The Working Group seeks feedback from the Web community on that specification and how well it furthers interoperability needs in the payments ecosystem. To provide feedback, see the status section above.
"""

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/browser-payment-api/issues/44