Linux: Embed AppImage update information

[shuvashish76]

When trying to update via AppImageUpdate

This is a common standard for updating AppImages.
AppImage having update information would also recognized by other software e.g. AppImageLauncher

info:

• https://github.com/AppImageCommunity/AppImageUpdate#update-information-overview
• https://github.com/AppImage/AppImageSpec/blob/master/draft.md#update-information

[c3er]

Thank you for reporting this issue!

I hope, that I manage to add these AppImage update information starting with the next version.

[shuvashish76]

Also do checkout this guide: https://github.com/ivan-hc/AppImage-tips