Add CONTRIBUTING document

CONTRIBUTING.md

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+# Contributing guide
+
+Thank you for your interest in contributing to Mailpit, your help is greatly appreciated! Please follow the guidelines below to ensure a smooth contribution process.
+
+
+## Code of conduct
+
+Please be respectful and considerate in all interactions. Mailpit is open source and free of charge, however is the result of thousands of hours of work.
+
+
+## Reporting issues and feature requests
+
+If you find a bug or have a feature request, please [open an issue](https://github.com/axllent/mailpit/issues) and provide as much detail as possible. Pleas do not report security issues here (see below).
+
+
+## Reporting security issues
+
+Please do not report security issues publicly in GitHub. Refer to [SECURITY document](https://github.com/axllent/mailpit/blob/develop/.github/SECURITY.md) for instructions and contact information.
+
+
+
+## How to contribute (pull request)
+
+1. **Fork the repository**  
+   Click the "Fork" button at the top right of this repository to create your own copy.
+
+2. **Clone your fork**  
+   ```bash
+   git clone https://github.com/your-username/mailpit.git
+   cd mailpit
+   ```
+
+3. **Create a branch**  
+   Use a descriptive branch name:
+   ```bash
+   git checkout -b feature/your-feature-name
+   ```
+
+4. **Make your changes**  
+   Write clear, concise code and include comments where necessary.
+
+5. **Test your changes**  
+   Run all tests to ensure nothing is broken. This is a mandatory step as pull requests cannot be merged unless they pass the automated testing.
+
+6. **Ensure your changes pass linting**  
+   Ensure your changes pass the [code linting](https://mailpit.axllent.org/docs/development/code-linting/) requirements. This is a mandatory step as pull requests cannot be merged unless they pass the automated linting tests.
+
+7. **Commit and push**  
+   Write a clear commit message:
+   ```bash
+   git add .
+   git commit -m "Describe your changes"
+   git push origin feature/your-feature-name
+   ```
+
+8. **Open a pull request**  
+   Go to your fork on GitHub and open a pull request against the `develop` branch. Fill out the PR template and describe your changes.
+
+---
+
+Thank you for helping make this project awesome!