OID4VP: add text explaining design rationale

[Torsten Lodderstedt]

In conversations with implementers, I often explain why we moved more logic to the verifier (e.g. the response endpoint) instead of using the code flow (with the token endpoint). We did that to foster privacy and need to explain that (as it causes more complexity for the verifier).

[Mark Haine]

Migrated to GitHub

https://github.com/openid/OpenID4VP