docs: describe `.github/org-wide-files-config.toml` in README

I was able to [reverse-engineer](https://github.com/exercism/java/pull/2706) how to make the Exercism bot stop removing the `fmt: true` setting in our configlet workflow on every PR by following https://github.com/exercism/org-wide-files/issues/308 -> https://github.com/exercism/nim/pull/528#issuecomment-1674312844, but it took some digging.

It would be nice if the README.md in this repository briefly mentions the fact that tracks can add the `.github/org-wide-files-config.toml` file and what options can be configured.