Lint for very short descriptions and write our own if not provided

A user wrote in that some families like [Tuffy](https://fonts.google.com/specimen/Tuffy/about?sort=date) and [Sensation](https://fonts.google.com/specimen/Sansation/about?sort=date) have very very short descriptions in their 'About' sections.

So I suggest we add a linter check for very short descriptions, prompting us to write our own descriptions if not provided by upstream. Say, a reg-ex for at least 3 sentences, and the link to the upstream repo.