chore(v3.1.2): add readme migration notice

[tiagosiebler]

Summary

Additional Information

PR Checklist

As part of the PR, make sure you have:

• No breaking changes / documented all breaking changes clearly.
• Updated & checked that all tests pass.
• Increased the version number in the package.json
• Checked npm install runs without issue.
• Included the package-lock.json, if it changed after npm install
• Checked npm run build runs without issue.
• Updated the endpoint map (optional, if you know how).
• Run llms.txt generator (optional, if you know how).