Added exercise reverse-string

[rainij]

Added a new practice exercise reverse-string from problem-specifications.

[rainij]

I essentially followed this procedure: https://exercism.org/docs/building/configlet/sync. I wonder about the file .docs/instructions.append.md which is present in a few exercises. It seems to contain track-specific text. I think I could use it here too. To express that one should not use List.reverse to solve the exercise (or something like that). At the moment I did not include it since I did not find any documentation on it. Can somebody point me in the right direction?

[ErikSchierboom]

At the moment I did not include it since I did not find any documentation on it. Can somebody point me in the right direction?

It's in the practice exercise docs: https://exercism.org/docs/building/tracks/practice-exercises#h-file-docs-instructions-append-md

[rainij]

After thinking a little bit about it I figured that .docs/instructions.append.md is not really necessary.