Add CI workflow and contribution guidelines

- GitHub Actions workflow for testing on Python 3.9-3.12
- Ruff linting in CI
- Coverage reporting to Codecov
- CONTRIBUTING.md with development setup instructions
- LICENSE file (MIT)
- Dev dependencies in pyproject.toml

CONTRIBUTING.md

@@ -0,0 +1,95 @@
+# Contributing to InvenTree RMA Plugin
+
+Thank you for your interest in contributing! This document provides guidelines for contributing to the project.
+
+## Getting Started
+
+1. Fork the repository
+2. Clone your fork:
+   ```bash
+   git clone https://github.com/YOUR_USERNAME/inventree-rma-plugin.git
+   cd inventree-rma-plugin
+   ```
+3. Install development dependencies:
+   ```bash
+   pip install -e ".[dev]"
+   ```
+
+## Development Workflow
+
+### Running Tests
+
+```bash
+pytest tests/ -v
+```
+
+### Running Linter
+
+```bash
+ruff check .
+```
+
+To auto-fix issues:
+```bash
+ruff check --fix .
+```
+
+### Code Style
+
+This project uses [ruff](https://github.com/astral-sh/ruff) for linting. Please ensure your code passes all checks before submitting a PR.
+
+## Pull Request Process
+
+1. Create a new branch for your feature/fix:
+   ```bash
+   git checkout -b feature/your-feature-name
+   ```
+
+2. Make your changes and commit with clear, descriptive messages
+
+3. Ensure all tests pass and linting is clean:
+   ```bash
+   pytest tests/ -v
+   ruff check .
+   ```
+
+4. Push to your fork and open a Pull Request
+
+5. Describe your changes in the PR description, including:
+   - What the change does
+   - Why it's needed
+   - Any breaking changes
+
+## Testing with InvenTree
+
+To test the plugin with a local InvenTree instance:
+
+1. Set up InvenTree using Docker:
+   ```bash
+   git clone https://github.com/inventree/InvenTree.git
+   cd InvenTree
+   docker compose -f contrib/container/docker-compose.yml up -d
+   ```
+
+2. Install the plugin in development mode:
+   ```bash
+   docker compose exec inventree-server pip install -e /path/to/inventree-rma-plugin
+   ```
+
+3. Enable the required settings in InvenTree:
+   - ENABLE_PLUGINS_APP
+   - ENABLE_PLUGINS_URL
+   - ENABLE_PLUGINS_INTERFACE
+
+## Reporting Issues
+
+When reporting issues, please include:
+- InvenTree version
+- Plugin version
+- Steps to reproduce
+- Expected vs actual behavior
+- Any error messages or logs
+
+## Questions?
+
+Feel free to open an issue for any questions about contributing.