Document existing rules (fixes #39)
Document existing rules (fixes #39)

I decided against generating the documentation from inline comments due to the
complexity of parsing correctly, and the brittleness of parsing simply (e.g.
regexes). This works for the moment.

Borgaard/markdownlint,peter1000/markdownlint,DavidAnson/markdownlint-ruby,mivok/markdownlint,markdownlint/markdownlint,exalted/markdownlint