docs(flatpak): explain skipping updater artifacts without a signing key

flatpak/README.md

@@ -24,6 +24,12 @@ flatpak install -y --user flathub org.gnome.Platform//46 org.gnome.Sdk//46
 pnpm tauri build --bundles deb
 ```
 
+The last step of this build signs the auto-update bundles, which requires the release private key `TAURI_SIGNING_PRIVATE_KEY`. Without the key, the `.deb` is still produced but the command exits with an error. To skip updater bundle generation instead:
+
+```bash
+pnpm tauri build --bundles deb -c '{"bundle":{"createUpdaterArtifacts":false}}'
+```
+
 2) Copy the generated deb to this directory:
 
 ```bash