docs: contributing instructions regarding versioning are out of date

The README on [versioning an exercise](https://github.com/exercism/haskell#exercise-versioning) seems to reflect the reality prior to "Reopening the Problem Specifications repo": https://github.com/exercism/problem-specifications/issues/1674.

From what I can see, versioning is still expected but:

- Exercises based on a canonical-data.json in [problem-specifications](https://github.com/exercism/problem-specifications) should use its version plus a serial number.

is no longer true and what was the fallback:

- Exercises that are not based on canonical-data.json should use version 0.1.0 plus a serial number.

is the true and only option.