flatpak: Add a message about the repository getting added with the app

When installing from a flatpakref file
2 jobs for !212 with work/fix-flatpakref-files in 1 minute and 1 second (queued for 4 seconds)
detached
Status Name Job ID Coverage
  Build
passed freebsd_qt515 #168595
FreeBSD

00:00:47

passed suse_tumbleweed_qt515 #168594
Linux

00:00:58