Fix publishing from GitHub Actions

Author: Dai MIKURUBE

README.md

@@ -115,3 +115,45 @@ formatter:
 ```
 $ ./gradlew gem
 ```
+
+For Maintainers
+----------------
+
+### Release
+
+Modify `version` in `build.gradle` at a detached commit, and then tag the commit with an annotation.
+
+```
+git checkout --detach master
+
+(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.
+
+```
+git push -u origin vX.Y.Z
+```

build.gradle

@@ -238,6 +238,10 @@ publishing {
 }
 
 signing {
+    if (project.hasProperty("signingKey") && project.hasProperty("signingPassword")) {
+        logger.lifecycle("Signing with an in-memory key.")
+        useInMemoryPgpKeys(signingKey, signingPassword)
+    }
     sign publishing.publications.maven
 }