Hints for gnome-software in main

Hints

• asv-nonstandard-gnome-extension
  :3540 - kudos
  This tag is a GNOME-specific extension to AppStream and not part of the official specification. Do not expect it to work in all implementations and in all software centers.
• asv-unknown-tag
  :3629 - developer
  Found invalid tag. Non-standard tags should be prefixed with `x-`. AppStream also provides the <custom/> tag to add arbitrary custom data to metainfo files. This tag is read by AppStream libraries and may be useful instead of defining new custom toplevel or `x-`-prefixed tags if you just want to add custom data to a metainfo file.