Create a Contribution Guide

To make it easier for new contributors to join the project, we need a clear CONTRIBUTING.md file that outlines the process for setting up the development environment and submitting changes.

This is a great non-code contribution for someone who wants to help with the project's community standards.

**Acceptance Criteria**

A new file named CONTRIBUTING.md is created in the project root.

The file should contain the following sections:

- Welcome Message: A brief, welcoming paragraph.
- How to Contribute: Instructions on how to find an issue (especially good first issue), fork the repository, create a new branch, and submit a pull request.
- Setting Up the Development Environment: A link to the "Development Setup" section of the README.md.
- Code Style & Linting: A brief mention that we use ruff for linting and formatting and that the CI will check this.
- Running Tests: The command to run the test suite (uv run pytest or pytest -v).