Hints for practice exercises?

Background on the [forum](https://forum.exercism.org/t/concept-exercise-files/13289/7).

We haven't generally included `docs/hints.md` for Practice exercises, though all our Concept exercises have them. As of today, only `fft` has hints, and `baffling birthdays` is about to become the second when the PR is merged.

I compared a few other tracks, to see how many they have (not many):
- Python 7
- JS none
- C# 1
- Elixir 6
- Java 2

I'm inclined to include them for new exercises, when the stumbling blocks are fresh in our mind. Adding them to 82 older exercises is more than I want to sign up for!

If we had some priorities for exercises that repeatedly trip up students, we could add Hints very much more easily than adding Approaches. I don't know who (if anyone!) does mentoring on the Julia track. They would probably have good suggestions, if we can find them.

The [build status page](https://exercism.org/tracks/julia/build) lists mentoring requests per exercise. That may be a useful clue, pointing us towards `rotational-cipher`, `secret-handshake`, `clock`.