Create CONTRIBUTING.md (#14)

[sql-hkr]

This pull request adds a comprehensive CONTRIBUTING.md file to the project, providing clear guidelines for new contributors. The document covers setup instructions, development workflows, testing, linting, documentation, and the pull request process, making it easier for others to contribute effectively.

Contributor onboarding and workflow documentation:

• Adds a detailed CONTRIBUTING.md with instructions for local development setup, including environment creation, dependency installation (with both uv and pip), and running tests.
• Outlines standards for testing, linting (using ruff), documentation updates, and example usage to ensure code quality and maintainability.
• Describes the preferred pull request process, including branch naming conventions, commit message guidelines, and a PR checklist to streamline code review.
• Includes sections on code of conduct, reporting security issues, and licensing to clarify community expectations and legal terms.