Document how users can choose their own shell

[runlevel5]

[tstromberg]

kernel.cafe has always supported a shell field in users.yaml, but no usage examples exist yet.

Here is the field: https://github.com/KernelCafe/automation/blob/484ac2b5e2c39701b37372fc526380c4c9046a56/cmd/generate-ansible/main.go#L36

Here is how it gets invoked:

https://github.com/KernelCafe/automation/blob/484ac2b5e2c39701b37372fc526380c4c9046a56/cmd/generate-ansible/main.go#L305

We should probably document this - so I'm going to leave this issue open for the time being.

[runlevel5]

@tstromberg what do you think about falling back to /bin/sh instead of fish?

[tstromberg]

It's sort of off-topic, but as a default, I have a strong preference for guiding users to a friendly modern shell. I'm open to alternatives but have a strong preference for a shell created within the last 20 years. fish seems to be a great fit. It also is slightly incompatible, which is a nice way to break behavioral assumptions