Add a Markdown linter

[sleevi]

Tools like markdownlint can help ensure consistency in the Markdown, which is especially important as each Chartered Working Group is free to produce their own documents however they want, and it's up to the Chairs to ensure consistency and formatting.

While there was discussion of a "Markdown style guide" in the past (now tracked at cabforum/forum#11 ), style guides benefit most from automated enforcement/checking (and, ideally, automated formatting).

[hablutzel1]

#51 fixes this.