xfce-do-release: Update Appdata file
This adds the new release to the Appdata file if the
exists. This file is only searched in the root of the repository and in
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)).