docs: add contributing guide

[2023Anita]

Summary

• Add a root CONTRIBUTING.md with local setup, test/check commands, and PR checklist.
• Link the new guide from the README contributing section.

Motivation

Closes #6.

Verification

• git diff --check
• Verified referenced files and package directories exist locally.
• pre-commit was not available in this local shell; this PR only changes Markdown files and the current pre-commit config contains the Black hook for Python files.

[2023Anita]

@microsoft-github-policy-service agree