document workflow to contribute exercises

Would be good to have a `CONTRIBUTING.md` documenting the workflow. Writing it down would also help me clarify the steps to myself.

As a first step, let's focus on practice exercises. My understanding so far

1. Choose an exercise and update `config.json`, adding at least the fields `name`, `difficulty`, `stub`. (I am a little surprised configlet cannot do this for me).
2. run `bin/configlet uuid` and add the code to the `uuid` field in `config.json`
3. run `bin/configlet sync --update --docs --metadata --tests --exercise $(EXERCISE-STUB)` to initialize the folder
4. initialize the folder. This could be done with `mason init`. The add `test/tests.chpl` (would be nice to automate with next step)
5. Looking at [problem specifications](https://github.com/exercism/problem-specifications/tree/main/exercises), add exercises to the tests folder.
6. write a reference solution in `.meta/reference.chpl`
7. from main directory, run `bin/test` to test the reference solution passes the tests.