Add flathub required metadata to appdata
Flathub requires OARS markup and release tags in the appdata file in order to more fully populate software stores like KDE Discover or the Flathub website. Without it, Kaidan can't be added to Flathub as the build fails.
OARS is super easy to do, you just follow the questionaire at https://odrs.gnome.org/oars and then paste the XML into the file.
Release tags should ideally be generated as part of the release process otherwise it's a bit of hassle. They need a version number and a date as a minimum, a changelog is optional but people like to see it.