Make TDD more test-driven by adding our linter and documentation

# Problem

`mix test.watch` allows you to add tasks beyond `test` to run. It would be helpful to also run our linter, `credo`. Because TDD is awesome.

## Subtasks
- [ ] Add `credo` to the tests run by `mix test.watch`; Cf. https://github.com/lpil/mix-test.watch
- [ ] Document use of `mix test.watch`
- [ ] Include documentation of `--stale` flag, which `test.watch` can receive directly. See https://hexdocs.pm/mix/Mix.Tasks.Test.html#module-stale