Update README.md before archiving (dart-archive/lints#214)

Author: mosuem

pkgs/lints/README.md

@@ -1,3 +1,6 @@
+> [!IMPORTANT]  
+> This repo has moved to https://github.com/dart-lang/core/tree/main/pkgs/lints
+
 [![Build Status](https://github.com/dart-lang/lints/workflows/validate/badge.svg)](https://github.com/dart-lang/lints/actions?query=branch%3Amain)
 [![pub package](https://img.shields.io/pub/v/lints.svg)](https://pub.dev/packages/lints)
 [![package publisher](https://img.shields.io/pub/publisher/lints.svg)](https://pub.dev/packages/lints/publisher)