Hints for appstream in main

Hints

• asv-content-rating-missing
  org.freedesktop.appstream.cli.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/
• asv-developer-info-missing
  org.freedesktop.appstream.cli.metainfo.xml -
  This component contains no `developer` element with information about its author.