add explanation of release steps

Author: hidakatsuya

README.md

@@ -107,6 +107,37 @@ And, launch electron on development:
 $ npm start
 ```
 
+## Releasing Editor
+
+This steps are for releasing a new version of Editor.
+
+### 1. Update to the new version
+
+The following two files need to be updated.
+
+- app/editor/version.js
+- app/package.json
+
+### 2. Update documents
+
+- CHANGELOG.md (required)
+- README.md (if needed)
+
+### 3. Push them to `build/release-x.x.x` branch and test the built packages for each platform
+
+Pushing to `build/*` branch runs a job to build the package. Make sure that the built packages for each platform work properly. You can download the packages from the artifact of the build. For more information, see [Build workflow](.github/workflows/build.yml).
+
+### 4. Create pullrequest
+
+If there is no problem, create a pullrequest.
+
+### 5. Merge pullrequest and push tag
+
+Pushing a tag runs a job to build and release package. Check the followings:
+
+- Correct version and content of the release
+- All package files are attached to the release
+
 ## License
 
 Thinreports Editor is licensed under the [GPLv3](https://github.com/thinreports/thinreports-editor/blob/master/GPLv3).