Document why others can't easily set up their own instances

[edunham]

It is currently not in a condition that allows people to easily set up their own instances.

To resolve, file issues documenting all reasons it's difficult for a clean instance to be set up. These will probably be along the lines of missing documentation and missing sample configurations.

I would like instructions that would allow a random member of the public to set up an instance, because I would like to know exactly how it works, because it will soon be my problem when it breaks.

[aidanhs]

Currently interested in the process for provisioning a new builder machine. In particular, where does $HOME/musl-root come from? https://github.com/rust-lang/rust-buildbot/blob/84126fdf6d0ccc4fbd477e91f25f61f1848539a1/master/master.cfg#L749