Add detail on roles in README

README should at least explain :
* how to write regex for the role matching (admin is a complete match, reader and writer is a regex, how is the domain retrieved ?)
* where the information is taken (access token ?) so that the user can configure its OIDC provider

Start with README and trace the existing role-matching and OIDC/access-token configuration references in the project. Document how exact and regex role matches work, how the domain is retrieved, and where role information comes from so users can configure their OIDC provider; done means these behaviors are explained in the README.

Written by the indexing model from the issue text.