docs: update README.md

Author: ansidev

README.md

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+# sample-gitflow-release-workflows
+
+Sample Gitflow release workflow using GitHub Actions.
+
+## Manual
+
+- Workflow definitions: [release.yaml](./.github/workflows/release.yaml)
+- You have to update two env variables: `BRANCH_MAIN`, `BRANCH_DEVELOP` if necessary.
+
+## License
+
+This source code is available under the [MIT License](/LICENSE).