first-time-setup information

[SimonBartels]

The readme to set up the VPN client for linux points to the cisco website but that doesn't work. (It requires a log-in that doesn't exist.) Instead the client can be downloaded from vpn.ku.dk.

[athas]

There is an ostensibly up-to-date guide here: https://github.com/diku-dk/howto/blob/main/vpn.md