Update README.md before archiving (#257)

README.md

diff --git a/README.md b/README.md
index 3044238..a0e2fe5 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,6 @@
+> [!IMPORTANT]  
+> This repo has moved to https://github.com/dart-lang/test/tree/master/pkgs/matcher
+
 [![Dart CI](https://github.com/dart-lang/matcher/actions/workflows/ci.yml/badge.svg)](https://github.com/dart-lang/matcher/actions/workflows/ci.yml)
 [![pub package](https://img.shields.io/pub/v/matcher.svg)](https://pub.dev/packages/matcher)
 [![package publisher](https://img.shields.io/pub/publisher/matcher.svg)](https://pub.dev/packages/matcher/publisher)