docs: add contributing guidelines (#122)

CONTRIBUTING.md

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+# Contributing to Sysdig Secure Jenkins Plugin
+
+Thank you for your interest in contributing to the Sysdig Secure Jenkins Plugin! We appreciate your time and effort in improving this project. Please follow the guidelines below to ensure a smooth contribution process.
+
+## Getting Started
+
+1. **Fork the Repository**: Start by forking the [Sysdig Secure Jenkins Plugin](https://github.com/jenkinsci/sysdig-secure-plugin) repository.
+2. **Clone Your Fork**:
+   ```sh
+   git clone https://github.com/YOUR_USERNAME/sysdig-secure-plugin.git
+   cd sysdig-secure-plugin
+   ```
+3. **Set Up Upstream Remote**:
+   ```sh
+   git remote add upstream https://github.com/jenkinsci/sysdig-secure-plugin.git
+   ```
+4. **Create a Branch**:
+   ```sh
+   git checkout -b feature-branch-name
+   ```
+
+## Development Setup
+
+This project uses **Maven** for build and dependency management. **Nix is recommended** but not required—it serves as a quality-of-life tool by providing a development environment with all necessary dependencies, including the correct Java version.
+
+### Using Nix (Optional)
+
+1. Ensure you have [Nix](https://determinate.systems/nix-installer/) installed.
+2. Enter the development shell:
+   ```sh
+   nix develop
+   ```
+   This provides a development environment with the necessary tools (Maven, Java, etc.).
+3. You can also build the plugin with:
+   ```
+   nix build
+   ```
+   The plugin will be compiled in `./result/sysdig-secure.hpi` 
+
+### Using Maven
+
+1. To build the plugin (it also runs the tests):
+   ```sh
+   mvn clean package
+   ```
+   The plugin will be compiled in `./target/sysdig-secure.hpi` 
+2. To just run tests:
+   ```sh
+   mvn clean verify
+   ```
+3. You can also execute Jenkins with the plugin installed so you can test it locally with:
+   ```
+   mvn hpi:run
+   ```
+
+### Using the Makefile
+
+The project includes a `Makefile` with useful commands:
+
+- **Update dependencies and parent POM:**
+  ```sh
+  make update
+  ```
+- **Run verification tests:**
+  ```sh
+  make verify
+  ```
+
+## Making Changes
+
+- The project follows the **Ports and Adapters (Hexagonal) Architecture**. Please ensure that your contributions respect this design pattern to maintain modularity and separation of concerns.
+
+- Use **Conventional Commits** for commit messages. See [Conventional Commits](https://www.conventionalcommits.org/) for guidelines.
+
+- Keep your changes focused on a single feature or fix.
+
+- Follow the existing code style and best practices.
+
+- Include meaningful commit messages.
+
+- Write tests if applicable.
+
+## Submitting a Pull Request
+
+1. Push your branch to your fork:
+   ```sh
+   git push origin feature-branch-name
+   ```
+2. Open a Pull Request (PR) against the `main` branch in the upstream repository.
+3. Ensure your PR description includes:
+    - A summary of changes
+    - Any related issue numbers (e.g., "Fixes #123")
+    - Steps to test the changes
+4. Wait for reviews and make necessary changes as requested.
+
+## Code Review and Merging
+
+- PRs require at least one approval from a maintainer.
+- Automated tests must pass before merging.
+- Squash commits if necessary to maintain a clean history.
+
+## Reporting Issues
+
+If you encounter any issues, please open a [GitHub Issue](https://github.com/jenkinsci/sysdig-secure-plugin/issues) with:
+
+- A clear title and description
+- Steps to reproduce the issue
+- Logs or error messages if applicable
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [Apache 2.0 License](http://opensource.org/licenses/Apache-2.0).
+
+---
+
+Thank you for your contributions! 🚀
+