The topmost .desktopfile in the root directory of the AppDir/AppImage should be enriched with additional lightweight metadata related to the AppImage.

http://standards.freedesktop.org/desktop-entry-spec/latest/ar01s11.html

PortableLinuxGames apparently already has defined some.

• X-AppImage-Name: Name of the application (e.g., Krita)
• X-AppImage-Version: Version of the application that is contained in the AppImage (e.g., 1.3.1)
• X-AppImage-Arch: Architecture of the AppImage (e.g., x86_64)
• X-AppImage-Release: Version or timestamp of the AppImage itself (added at AppImage creation time)
  ...

However. let's try not to be redundant to AppStream information. So let's not include the license, the URL, etc.