Update shibboleth auth documentation #693
Description
Since introducing jicofo.conf it seems that the documentation how to add shibboleth authentication is not anymore up to date: https://github.com/jitsi/jicofo/blob/master/doc/shibboleth.md
The documentation should be updated to clarify how shibboleth auth should work with the new jicofo.conf configuration
See related the question how to configure shibboleth auth in docker environments jitsi/docker-jitsi-meet#956