Reference Resource Indicators for OAuth 2.0

As [suggested](https://twitter.com/jankytweet/status/1006613055717847041) by Karl McGuinness, this specification should reference the [Resource Indicators for OAuth 2.0](https://tools.ietf.org/html/draft-campbell-oauth-resource-indicators-02) specification regarding the use of the `resource` parameter in both authorization and token requests. With that reference in place, the scope of this specification would be narrowed to focus on the `issuer` parameter in the HTTP authentication challenge, as well as the requesting party role.

No file or test is named. Locate the draft text and its references, then compare the OAuth 2.0 Resource Indicators specification with the current treatment of the resource parameter in authorization and token requests. Done means the reference is added and the draft's scope clearly focuses on the issuer parameter and requesting party role.

Written by the indexing model from the issue text.