Add note about cupertino_http state in README (#755)

Author: mit-mit

pkgs/cupertino_http/README.md

@@ -4,6 +4,23 @@
 A macOS/iOS Flutter plugin that provides access to the
 [Foundation URL Loading System][].
 
+## Status: Experimental
+
+**NOTE**: This package is currently experimental and published under the
+[labs.dart.dev](https://dart.dev/dart-team-packages) pub publisher in order to
+solicit feedback. 
+
+For packages in the labs.dart.dev publisher we generally plan to either graduate
+the package into a supported publisher (dart.dev, tools.dart.dev) after a period
+of feedback and iteration, or discontinue the package. These packages have a
+much higher expected rate of API and breaking changes.
+
+Your feedback is valuable and will help us evolve this package. For general
+feedback, suggestions, and comments, please file an issue in the 
+[bug tracker](https://github.com/dart-lang/http/issues).
+
+## Motivation
+
 Using the [Foundation URL Loading System][], rather than the socket-based
 [dart:io HttpClient][] implemententation, has several advantages: