Update README with information for maintainers

Author: hiroyuki-sato

README.md

@@ -1,6 +1,6 @@
-# Command file output plugin for Embulk
+# embulk-output-command
 
-This plugin runs a command and writes formatted data to its stdin.
+Command file output plugin for Embulk: runs a command and writes formatted data to its stdin.
 
 ## Overview
 
@@ -57,8 +57,44 @@ out:
     type: csv
 ```
 
-## Build
+For Maintainers
+----------------
+
+### Release
+
+Modify `version` in `build.gradle` at a detached commit, and then tag the commit with an annotation.
+
+```
+git checkout --detach main
+
+(Edit: Remove "-SNAPSHOT" in "version" in build.gradle.)
+
+git add build.gradle
+
+git commit -m "Release vX.Y.Z"
+
+git tag -a vX.Y.Z
+
+(Edit: Write a tag annotation in the changelog format.)
+```
+
+See [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) for the changelog format. We adopt a part of it for Git's tag annotation like below.
+
+```
+## [X.Y.Z] - YYYY-MM-DD
+
+### Added
+- Added a feature.
+
+### Changed
+- Changed something.
+
+### Fixed
+- Fixed a bug.
+```
+
+Push the annotated tag, then. It triggers a release operation on GitHub Actions after approval.
 
 ```
-$ ./gradlew gem
+git push -u origin vX.Y.Z
 ```