appdata: add release tag

The description is not filled in. We can't merge this with the NEWS file
because this pertains only to the editor.

There's no machinery to automatically check this yet. Just the bare
minimum to keep the new version of appdata-util happy.
6 jobs for lr/appdata in 17 minutes and 51 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #461139
fedora28_dist

00:06:58

 
  Test
passed #461144
centos7

00:10:51

passed #461143
fedora_autotools_full

00:05:32

passed #461142
fedora_autotools_minimal

00:02:32

passed #461141
fedora_meson_full

00:10:09

passed #461140
fedora_meson_minimal

00:07:32