library criteria: Must be used by Circle/Core projects

Rephrase the criterium to ensure the library is used by a project in Circle or Core. That way, we avoid libraries that are only used by a random abandoned projects from 2 years ago, as well as ensure the library is actually appealing to developers, making for a higher barrier of entry for library devs.

Please comment or give a thumbs up/down, so I know when to merge this.