add hints.md

[depial]

Ports the hints.md from the Julia track.

Notes:

• Not necessary, but maybe consider moving up the last task 8. Check the size of the stack since other tasks appear to use length (e.g. 5. Remove the top card from the stack.
• A link to length could be included, but I neglected to since that was the only possible link.
• Fixed a small typo (missing parenthesis) in introduction.md.

Everything else is pretty straightforward for a straightforward exercise.

[colinleach]

consider moving up the last task

Yes, good idea.

[depial]

I've moved Task 8 to Task 5, and bumped everything else down. I hope I've made all the adjustments in all the necessary docs...