Point to new repo location in README

Author: stuartmorgan-g

pkgs/platform/README.md

@@ -1,5 +1,12 @@
 [![Pub](https://img.shields.io/pub/v/platform.svg)](https://pub.dartlang.org/packages/platform)
 
+## Moved
+
+The source for the `platform` package has moved to [the `flutter/packages`
+repository](https://github.com/flutter/packages/tree/main/packages/platform).
+
+-----
+
 A generic platform abstraction for Dart.
 
 Like `dart:io`, `package:platform` supplies a rich, Dart-idiomatic API for