feat: add documented local test flow

github-actions/release-prepare/.env.example

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+GITHUB_TOKEN=PASTE_YOUR_TOKEN
+GITHUB_REPOSITORY=exampleorg/exampleproject
+RELEASE_BRANCH=main
+GITHUB_REF_NAME=develop

github-actions/release-prepare/.gitignore

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+.env

github-actions/release-prepare/README.md

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+# GitHub Action: release-prepare
+
+## Development
+
+This action encapsulates most of its complexity in a standalone Bash script that accepts environment variables, making it easy to test locally with the help of a `.env` file you can point at a test repository.
+
+1. Create `.env` from the template:
+
+    ```bash
+    cp .env.example .env
+    ```
+
+2. Paste a GitHub token and tailor repository/branches to your test target.
+
+3. Run bash script with `.env` applied to emulate GitHub Actions context:
+
+    ```bash
+    eval $(< .env) ./pull-request.sh
+    ```