new end_session endpoint

Add a small "migration guide" to the README.md on how to convert from logout endpoint to end_session endpoint.

I have an own OIDC plugin that derives from the openid_connect base client, the logout functionality stopped working properly as the new openid_connect version in dev-3.x has an "end_session" endpoint, which we didn't have before.

As workaround I copied the logout endpoint to the end_session endpoint in the config of my plugin, and everything works again.