Add contributing guide

CONTRIBUTING.md

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+# Contributing to Chronos
+
+Thank you for your interest in contributing to Chronos! This document provides guidelines and instructions for contributing.
+
+## Getting Started
+
+1. Fork the repository
+2. Clone your fork:
+   ```bash
+   git clone https://github.com/YOUR_USERNAME/chronos.git
+   cd chronos
+   ```
+3. Install dependencies:
+   ```bash
+   composer install
+   ```
+
+## Running Tests
+
+Run the test suite using PHPUnit:
+
+```bash
+composer test
+```
+
+Or directly:
+
+```bash
+vendor/bin/phpunit
+```
+
+## Code Style
+
+This project follows the [CakePHP coding standards](https://book.cakephp.org/5/en/contributing/cakephp-coding-conventions.html).
+
+Check for coding standard violations:
+
+```bash
+composer cs-check
+```
+
+Automatically fix coding standard violations:
+
+```bash
+composer cs-fix
+```
+
+## Static Analysis
+
+PHPStan is used for static analysis. First, install the tools:
+
+```bash
+composer stan-setup
+```
+
+Then run the analysis:
+
+```bash
+composer stan
+```
+
+## Running All Checks
+
+To run tests, coding standards, and static analysis together:
+
+```bash
+composer check
+```
+
+## Submitting Changes
+
+1. Create a new branch for your changes
+2. Make your changes and commit them with clear, descriptive messages
+3. Ensure all checks pass (`composer check`)
+4. Push to your fork and submit a pull request
+
+## Reporting Issues
+
+- Search existing issues before creating a new one
+- Include PHP version, Chronos version, and a minimal reproduction case
+- Use clear, descriptive titles
+
+## Documentation
+
+Documentation source files are located in the `docs/` directory. The documentation is published at [book.cakephp.org/chronos](https://book.cakephp.org/chronos/3/en/).