docs(repo): add readme retirement banner

[gibbardsteve]

📌 README retirement banner

✨ Summary

Add a banner to say the repo will be retired

📜 Changes Introduced

• documentation (docs:)
• Updates to tests and/or documentation

✅ Checklist

Please confirm you've completed these checks before requesting a review.

• Code is formatted using Black
• Imports are sorted using isort
• Code passes linting with Ruff, Pylint, and Mypy
• Security checks pass using Bandit
• API and Unit tests are written and pass using pytest
• DocStrings follow Google-style and are added as per Pylint recommendations
• Documentation has been updated if needed

🔍 How to Test

Manual review