I don't watch Homebrew/homebrew-core any more (as other maintainers
are doing a great job of the formula work without my help) but I do
want to ensure that I keep on top of changes to certain files.
As a result, add a `CODEOWNERS` file so I can be requested for review on
changes to these files:
https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners