Add release workflow and document

Signed-off-by: morimoto-cybozu <[email protected]>

.github/workflows/release.yaml

@@ -0,0 +1,20 @@
+name: release
+on:
+  push:
+    tags:
+    - 'v*'
+jobs:
+  release:
+    name: Release on GitHub
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+      - name: Create release
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          version=$(echo ${{ github.ref }} | sed -e "s#refs/tags/##g")
+          if echo $version | grep -q -e '-'; then prerelease=-p; fi
+          gh release create $version $prerelease \
+            -t "Release $version"  \
+            -n "See [CHANGELOG.md](./CHANGELOG.md) for details."

RELEASE.md

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+Release procedure
+=================
+
+This document describes how to release a new version of log.
+
+Versioning
+----------
+
+Follow [semantic versioning 2.0.0][semver] to choose the new version number.
+
+Prepare change log entries
+--------------------------
+
+Add notable changes since the last release to [CHANGELOG.md](CHANGELOG.md).
+It should look like:
+
+```markdown
+(snip)
+## [Unreleased]
+
+### Added
+- Implement ... (#35)
+
+### Changed
+- Fix a bug in ... (#33)
+
+### Removed
+- Deprecated `-option` is removed ... (#39)
+
+(snip)
+```
+
+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. Make a branch to release
+
+    ```console
+    $ git neco dev "bump-$VERSION"
+    ```
+
+3. Edit `CHANGELOG.md` for the new version ([example][]).
+4. Commit the change and push it.
+
+    ```console
+    $ git commit -a -m "Bump version to $VERSION"
+    $ git neco review
+    ```
+
+5. Merge this branch.
+6. Add a git tag to the main HEAD, then push it.
+
+    ```console
+    # Set VERSION again.
+    $ VERSION=x.y.z
+    $ echo $VERSION
+
+    $ git checkout main
+    $ git pull
+    $ git tag -a -m "Release v$VERSION" "v$VERSION"
+
+    # Make sure the release tag exists.
+    $ git tag -ln | grep $VERSION
+
+    $ git push origin "v$VERSION"
+    ```
+
+GitHub actions will build and push artifacts such as container images and
+create a new GitHub release.
+
+[semver]: https://semver.org/spec/v2.0.0.html
+[example]: https://github.com/cybozu-go/etcdpasswd/commit/77d95384ac6c97e7f48281eaf23cb94f68867f79