Key provider registration details lacking

[ianbjacobs]

Creating an individual issue from the list in #26 in reference to:
https://github.com/w3c/webpayments-methods-tokenization/wiki/Tokenized-Card

"With respect to "Token Service Providers MAY require KeyProviders to register/onboard." - register with a third-party service such a clearinghouse, with the Token Service Provider itself, or both? What is the purpose of such registration (e.g., so that the Token Service Provider can restrict its interactions to trusted entities)?"

[ianbjacobs]

Discussed on 30 January [1]. As a result began a FAQ entry on establishment of trust.

[1] https://www.w3.org/2018/01/30-wpwg-minutes#item05