chore(ci): Add markdown linter

[kakkoyun]

          GitHub's UI is not so great for reviewing prose (you can't comment on just certain words; have to comment on the full line).

It can make things a little easier to flow documents at a fixed width, so long sentences are broken down on multiple lines...

I'd suggest (completely optional) flowing Markdown at 100 columns (or 80; but I usually do 100 myself).
This means just putting a hard-line-break before the 1st word that crosses the line.
Many IDEs have auto-formatters that'll help do this automatically; and we could enforce this with markdownlint or similar.

Originally posted by @RomainMuller in #14 (comment)

[ralf0131]

Looks good to me, feel free to send a pull request.