README: Replace outdated information about hints.md with correct information

[petertseng]

It is used together with the respective description.md for the exercise from 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 Remove this outdated information.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