release build workflow

README.md

@@ -69,7 +69,13 @@ make check     # bash -n on all scripts, plus shellcheck if installed
 make deb       # build/terraria-server-manager_<version>_all.deb
 ```
 
-To release: update `VERSION` and `CHANGELOG.md`, run `make deb`, and attach the `.deb` to the release.
+To release, update `VERSION` and `CHANGELOG.md`, commit, and push a matching tag:
+
+```bash
+git tag v$(cat VERSION) && git push origin v$(cat VERSION)
+```
+
+The `release` workflow (`.gitea/workflows/release.yaml`, Gitea Actions) then builds the `.deb` and publishes it as a release, with that version's changelog section as the notes. It needs a runner with the `ubuntu-latest` label. To build a package by hand instead, run `make deb`.
 
 ## Disclaimer