README: Replace outdated information about hints.md with correct information

> It is used together with the respective `description.md` for the exercise from [problem-specifications](https://github.com/exercism/problem-specifications) to build the `README.md` file.

This is just wrong. Remove this outdated information. Replace it with the actual purpose of hints.md. You can take it from https://github.com/exercism/docs/blob/main/building/tracks/presentation.md:

provide hints to a student to help them get themselves unstuck in an exercise