ought to contain info about `make memcheck`

I just finished linked-list, my first exercise with malloc and free, and I was looking for instructions to check for leaked memory. I ended up on the track docs testing page which is kind of out of the way.

Are maintainers open to a PR for adding some words about make memcheck to exercises/shared/.docs/

You might want to ask this question in an issue on GitHub.
Of the four maintainers I’ve only seen @bobahop here on the forum.

Good tip.