Arch is down for the summer

[seanfisk]

Arch being down for the summer should be explained in the docs.

Also, for the SSH keys, instead of saying "down for maintenance", it should say "down for the summer" (I think). Saying that all the machines are "down for maintenance" implies that it is temporary and they will be up once the maintenance is done, which is not true.

Alternatively, remove the Arch machines from /etc/hosts for the summer, which our build system uses.