docs: add release notes

RELEASE.md

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+# Release
+
+As of v31, releases of this action follow Semantic Versioning.
+
+### Publishing a new release
+
+#### Publish the release
+
+Draft [a new release on GitHub](https://github.com/cachix/install-nix-action/releases):
+
+- In `Choose a tag`, create a new tag, like `v31.2.1`, following semver.
+- Click `Generate release notes`.
+- `Set as the latest release` should be selected automatically.
+- Publish release
+
+#### Update the major tag
+
+The major tag, like `v31`, allows downstream users to opt-in to automatic non-breaking updates.
+
+This process follows GitHub's own guidelines:
+https://github.com/actions/toolkit/blob/main/docs/action-versioning.md
+
+##### Fetch the latest tags
+
+```
+git pull --tags --force
+```
+
+##### Move the tag
+
+```
+git tag -fa v31
+```
+```
+git push origin v31 --force
+```
+
+#### Update the release notes for the major tag
+
+Find the release on GitHub: https://github.com/cachix/install-nix-action/releases
+
+Edit the release and click `Generate release notes`.
+Edit the formatting and publish.
+