MALTDS-142: Document Kerberos ccache collection

After the SSO presentation at vCHEP, there have been some discussions about Kerberos and how to mix CERN and external institutes ccaches, so there would be no need to do a manual switch between them. The MIT Kerberos ccache collection feature enables automatic selection for the ccache to use depending of the service to authenticate against.

mentioned in commit 5af6233f

merged

mentioned in merge request !60 (merged)