Add CONTRIBUTING.md for contribution guidelines

This document outlines the contribution guidelines for the firmware targeting a modified Creality Ender-3 platform, including project scope, supported hardware, workflow, design principles, code guidelines, testing requirements, and documentation.

Author: DarkNinja-Lab

CONTRIBUTING.md

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+# Contributing
+
+This firmware targets a modified Creality Ender-3 platform.
+Contributions are welcome if they improve reliability, print quality, or maintainability.
+
+## Project Scope
+- Custom Ender-3 firmware
+- Hardware-near control logic
+- Motion, temperature, and safety handling
+- No generic “one-size-fits-all” support
+
+## Supported Hardware
+- Creality Ender-3 (modified)
+- Documented stepper drivers, sensors, and boards only
+- Changes must not assume stock hardware
+
+## Contribution Workflow
+1. Fork the repository
+2. Create a focused branch (`feature/*`, `fix/*`, `tuning/*`)
+3. Implement and test your change
+4. Open a Pull Request with:
+   - Hardware setup
+   - Test results
+   - Risks or side effects
+
+## Design Principles
+- Safety over features
+- Deterministic behavior
+- Explicit configuration
+- No hidden magic
+
+## Code Guidelines
+- Clear, hardware-oriented naming
+- No hardcoded machine-specific values
+- Avoid timing hacks and busy-waits
+- Respect real-time constraints
+- Comments explain *why*, not *what*
+
+## Testing Requirements
+- Compile must succeed without warnings
+- Test on real hardware (no blind merges)
+- Validate:
+  - Homing
+  - Thermal protection
+  - Motion limits
+- Include logs, measurements, or prints if relevant
+
+## What Will Not Be Accepted
+- Untested changes
+- Feature requests without hardware context
+- Changes that reduce safety margins
+- Abstract refactors without measurable benefit
+
+## Configuration
+- New settings must be documented
+- Defaults must be safe
+- Breaking config changes require migration notes
+
+## Documentation
+- Update README if behavior changes
+- Firmware behavior must be reproducible
+- Hardware assumptions must be explicit
+
+## License
+By contributing, you agree that your contributions are licensed under the project license.