Merge pull request dart-archive/appengine#150 from dart-lang/mit-mit-patch-1

Update README.md

Author: mit-mit

pkgs/appengine/README.md

@@ -4,6 +4,20 @@ This package provides support for running server applications written in Dart on
 [Google App Engine](https://cloud.google.com/appengine/) using
 [Custom Runtimes with Flex Environment](https://cloud.google.com/appengine/docs/flexible/custom-runtimes/).
 
+## 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/appengine/issues).
 
 ## Prerequisites