Add support for adding releases

[Jacalz]

It would be great to support adding releases. This is a requirement to publish a Flatpak application. Some useful fields to support would be description and url.

Example:
https://github.com/Jacalz/rymdport/blob/9bfedcf17e17eee7eff505b18543987740e66eae/internal/assets/unix/io.github.jacalz.rymdport.appdata.xml#L39

[ximion]

Patches welcome! ;-)