Describe why basic issuance is insufficient for rate limiting

[chris-wood]

One could imagine trying to implement rate limiting with the basic issuance protocol, e.g., by using an attester trusted by the origin to issue a fixed amount of tokens per time period. However, in this model, the attester would necessarily learn the origin information, breaking per-client and per-server separation. We should add more text motivating the attester/issuer split, and the motivation for the current design.

[tfpauly]

@dvorak42 @smhendrickson suggested also adding informative references to W3C work

[smhendrickson]

Notes to self: We should directly point to the combined attestor-issuer model for cases that don't have these constraints.

[tfpauly]

Probably should be a paragraph in motivation section