xfce-do-release: Update Appdata file

This adds the new release to the Appdata file if the <releases> tag exists. This file is only searched in the root of the repository and in the data directory, and must be unique.

I don't know if it's worth adding this by default (or silently?). I will personally use it for Mousepad after it is made available on Flathub (see apps/mousepad#146 (closed)).