Repository: Add release instructions

RELEASING.md

@@ -0,0 +1,13 @@
+<!--
+SPDX-FileCopyrightText: Night Theme Switcher Contributors
+SPDX-License-Identifier: CC-BY-SA-4.0
+-->
+
+# Releasing
+
+Release checklist:
+
+- Bump version number in [`meson.build`](./meson.build)
+- Add entry in [`CHANGELOG.md`](./CHANGELOG.md)
+
+To make a release, create a new tag. The CI will automatically add release notes from the changelog.