Hints for gnome-books in main

Hints

• metainfo-validation-hint
  Validation of the metainfo file yielded the following hint: Consider using a secure (HTTPS) URL for 'http://www.gnome.org/friends'
• metainfo-validation-hint
  Validation of the metainfo file yielded the following hint: Found invalid tag: 'kudos'. This tag is a GNOME-specific extension to AppStream and is not supported by all implementations.