clarify why we use the ruby version

 * [linkchecker](https://github.com/linkchecker/linkchecker/): can check links in HTML (anarcat is one of the

   maintainers), has many alternatives, see for example [lychee](https://github.com/lycheeverse/lychee)

Note that we currently use [markdownlint][] (the Ruby version, not the

Nodejs one, because anarcat dislikes nodejs more than Ruby), but we

could expand this to cover more things.

Note that we currently use [markdownlint][], the Ruby version, not the

Node version. This was primarily because anarcat dislikes Node more

than Ruby, but it turns out the Ruby version also has more

features. Notably, it can warn about Kramdown compilation errors, for

example finding broken Markdown links.

See also [this LWN article](https://lwn.net/Articles/822969/).