[Feature]: Create CONTRIBUTING.md guide for new contributors

[rahuldass19]

Which engine does this affect?

Documentation

Is this feature deterministic?

No - Requires LLM reasoning

Problem Statement

We need a clear contribution guide for new developers.

Proposed Solution

Should include

• How to set up local environment
• How to run tests
• Code style guidelines
• How to submit PRs

Alternatives Considered

No response

Checklist

• I have read CONTRIBUTING.md
• This is NOT an enterprise feature (audit, SSO, RBAC)