Update Reference Implementation README.md to identify current version as a POC

[afogel]

What changed

Update README to communicate reference implementation maturity

Type of change

• Specification change (schema, hooks, events, AgBOM)
• Documentation
• Tooling or CI
• Governance (licensing, security policy, contributor docs)

Checklist

• Commits are signed off with git commit -s (required by the DCO)
• Prose follows STYLE.md
• uv run mkdocs build --strict passes
• No secrets, tokens, or internal URLs in the diff

Security

• This change has no security impact