docs: add contributor README file

CONTRIBUTING.md

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+### Contributing
+
+Thank you for your interest in contributing to this project! We welcome and appreciate all forms of contribution—bug fixes, new features, documentation improvements, and tests. Following these guidelines helps ensure a smooth and efficient review process.
+
+---
+
+### Before You Start
+
+1. **Search Existing Work**  
+   Check the repository’s [issues] and [pull requests] to see if someone is already working on the same idea or problem.
+
+2. **Discuss Large Changes**  
+   For major changes or new features, open an [issue] first to discuss the design, scope, and implementation plan with the maintainers.
+
+3. **Read the Code of Conduct**  
+   Please review and follow our [Code of Conduct](https://github.com/veraison/corim/blob/main/CODE_OF_CONDUCT.md). We expect all contributors to maintain respectful and inclusive behavior.
+
+---
+
+### How to Contribute
+Follow the steps below to prepare and submit your contribution:
+
+### 1. Fork & Clone
+Start by forking the repository to your GitHub account and cloning it locally:
+
+```bash
+git clone https://github.com/<your-username>/corim.git
+cd corim
+```
+
+### 2. Set Up the Project
+Refer to the project setup instructions in the `README.md` (or the repository's setup documentation) to install dependencies and configure your local environment.
+
+### 3. Create a Branch
+Create a new, descriptively named branch for your work. Use one of the recommended prefixes:
+
+`feature/` for new features.
+
+`fix/` for bug fixes.
+
+`chore/` for maintenance or build-related tasks.
+
+```bash
+git checkout -b feature/your-feature-name
+```
+
+### 4. Make Changes
+Keep it Focused: Where possible, keep your changes focused on a single concern.
+
+Adhere to Standards: Follow the project's coding style and conventions.
+
+Document Code: Keep your code well-documented and include clear inline comments where complex logic is involved.
+
+### 5. Add or Update Tests
+Where applicable, add tests that fully cover your changes. New or updated tests are crucial for preventing regressions and significantly expedite the review process.
+
+### 6. Run Tests
+Before submitting your contribution, ensure the entire test suite passes successfully using the project's designated test command:
+
+```bash
+make test
+```
+
+### 7. Commit Your Changes
+Write clear, concise, and descriptive commit messages.
+
+Example:
+
+```bash
+git add .
+git commit -m "Fix: handle null pointer in data loader (fixes #42)"
+```
+
+### 8. Push Your Changes
+Push your new branch to your forked repository:
+
+```bash
+git push origin feature/your-feature-name
+```
+
+### 9. Create a Pull Request (PR)
+Open a pull request from your forked branch to the main repository's target branch (usually main or master).
+
+Detailed Description: Include a comprehensive description of your changes, the rationale behind them, and any relevant issue numbers (e.g., Closes #101).
+
+Checklist: Consider including a small checklist in your PR description (e.g., tests added/updated, documentation updated, code style checked).