Add release.md

docs/relese.md

@@ -0,0 +1,35 @@
+Release procedure
+=================
+
+This document describes how to release a new version.
+
+## Labeling
+
+Release notes are automatically generated based on PRs included in the release.
+Those PRs are categorized based on the label assigned to them.
+Please refer to `.github/release.yml` for the kind of labels.
+
+## Versioning
+
+Follow [semantic versioning 2.0.0][semver] to choose the new version number.
+
+## Bump version
+
+1. Determine a new version number. Then set `VERSION` variable.
+
+    ```console
+    # Set VERSION and confirm it. It should not have "v" prefix.
+    $ VERSION=x.y.z
+    $ echo $VERSION
+    ```
+
+2. Add a git tag to the main HEAD, then push it.
+
+    ```console
+    $ git checkout main
+    $ git tag -a -m "Release v$VERSION" "v$VERSION"
+    $ git tag -ln | grep $VERSION
+    $ git push origin v$VERSION
+    ```
+
+[semver]: https://semver.org/spec/v2.0.0.html