Add Markdown Explanation Docs

[MondayMorningHaskell]

A good alternative to having all the explanations in comments in the exercise files would be to make a Markdown file for each exercise (or possibly each category of exercises, i.e. one for each sub-directory). These files would contain the exact text from the explanation blocks except formatted to Markdown. There should also be a link in the exercise file to the Markdown and vice-versa. There should also be a link in each Markdown document to the next explanation document.

Don't feel the need to do all the exercises at once. Entirely OK to do a few at a time.