Hints for screenkey in main

Hints

• asv-description-first-para-too-short
  org.thregr.screenkey.metainfo.xml:13 - A screencast tool to display your keys.
  The first `description/p` paragraph of this component might be too short (< 80 characters). Please consider starting with a longer paragraph to improve how the description looks like in software centers and to provide more detailed information on this component immediately in the first paragraph.
• asv-content-rating-missing
  org.thregr.screenkey.metainfo.xml -
  This component has no `content_rating` tag to provide age rating information. You can generate the tag data online by answering a few questions at https://hughsie.github.io/oars/