lint: Add reminder to add contributors to `.meta/config.json`

From @iHiD on Slack:

> You'll also notice that each exercise has a config.json file: https://github.com/exercism/v3/blob/master/languages/csharp/exercises/concept/strings/.meta/config.json
> This file will do various things, but for now it lists the contributors to an exercise.
> This services two purposes:
> 1) Contributors will get kudos next to that exercise on the Exercism website, along with Credits and Reputation
> 2) Contributors will get assigned/notified for future PRs to the exercise, along with the codeowners.
> So remember to fill this out if someone makes a significant contribution to an exercise.

It would be great to have a workflow that adds a reminder if a contributor adds to an exercise but is not listed in the file yet. It should **never** fail, only post a comment, e.g. with [actions/github-script](https://github.com/actions/github-script)