Update README.md before archiving (#389)

README.md

diff --git a/README.md b/README.md
index 3ff0637..ba33079 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,6 @@
+> [!IMPORTANT]  
+> This repo has moved to https://github.com/dart-lang/http/tree/master/pkgs/web_socket_channel
+
 [![CI](https://github.com/dart-lang/web_socket_channel/actions/workflows/test-package.yml/badge.svg?branch=master)](https://github.com/dart-lang/web_socket_channel/actions/workflows/test-package.yml)
 [![pub package](https://img.shields.io/pub/v/web_socket_channel.svg)](https://pub.dev/packages/web_socket_channel)
 [![package publisher](https://img.shields.io/pub/publisher/web_socket_channel.svg)](https://pub.dev/packages/web_socket_channel/publisher)
@@ -70,4 +73,4 @@
 connects to a listening server using the appropriate implementation for the
 platform.
 
-[connect]: https://pub.dev/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/WebSocketChannel.connect.html
\ No newline at end of file
+[connect]: https://pub.dev/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/WebSocketChannel.connect.html