docs: add contributing guide

[b1rdmania]

Summary

• add a lightweight CONTRIBUTING.md for Mike contributors
• document local setup and relevant checks
• encourage small, focused PRs that are easier to review
• call out safe local testing with disposable infrastructure and synthetic documents
• remind contributors not to include secrets, client data, or sensitive legal documents

Validation

• git diff --check

Documentation-only change.

[willchen96]

Thank you for proposing this. I included a contributing.md in PR #139 with some ideas here included.