docs: add contribution guidelines

[MugdhoAI]

Summary

• Document the actual make and make test workflow.
• Add project-specific review expectations for B-tree invariants, persistence, malformed files, I/O errors, and sanitizer checks.
• Clarify benchmark limitations and file-format compatibility.

Verification

• Checked build and test commands against the repository Makefile and existing MIT license.
• Documentation-only change; no application code changed.