Add a contributor quickstart path to the README

[Permea-lab-admin]

Goal

Make it easier for a new contributor to understand where to start in Permea Core.

Suggested scope

• Add a short Contributor quickstart section to README.md or improve the existing contributor pointer.
• Link to the most relevant files for first-time contributors: CONTRIBUTING.md, docs/OSS_OPERATING_DOCS_MAP.md, and docs/ROADMAP.md.
• Keep the section concise.

Acceptance criteria

• A newcomer can identify 2-3 good first places to read.
• The links point to existing files.
• The change does not alter project scope or governance policy.

Contributor note

This is a documentation-only issue intended for a first PR.