Add documentation about "what doesn't make a good track"

[kytrinyx]

Only the newsletter has this: http://tinyletter.com/exercism/letters/what-exercism-is-and-isn-t ... we need to add it somewhere, but not sure where.

I think it probably needs its own file, probably within the about/ directory. Maybe call it in-scope-out-of-scope.md

[OrthoDex]

Hey @kytrinyx ! I would like to take this up if it's alright.

[kytrinyx]

Hi @OrthoDex, yes! That would be wonderful. Thank you 🌻