So now it can be used as a pre-commit hook, validating and nicely
formatting YAML files. Hurray!

So now this is sort-of Black for YAML files. Next, pre-commit hooks.

Let's say a naive implementation. Mainly to mimic the json.tool format
generally (most of the switches are not applicable and I'm not going to
mimic it completely).

- README.
- License.
- .gitignore.
- pre-commit config.
- GitLab CI config.