Add comprehensive documentation and code comments

This commit adds extensive documentation to the Mistral OCR CLI project:

- Add API.md with detailed API response format documentation
- Add CHANGELOG.md to track version changes
- Add CONTRIBUTING.md with guidelines for contributors
- Enhance README.md with more detailed usage examples and troubleshooting
- Add proper docstrings to all Python modules and functions
- Update requirements.txt with development dependencies
- Improve setup.py with better metadata

These changes make the project more accessible to users and contributors.

requirements.txt

@@ -1 +1,9 @@
+# Core dependencies
 requests>=2.25.0
+
+# Optional dependencies for enhanced functionality
+markdown>=3.3.0  # For converting markdown to HTML
+pytest>=6.0.0    # For running tests
+pytest-cov>=2.0.0  # For test coverage reports
+black>=21.0.0    # For code formatting
+flake8>=3.9.0    # For linting