clarify flutter is early-stage

README.md

@@ -6,6 +6,11 @@ Flutter is a new way to build high-performance, cross-platform mobile apps.
 Flutter is optimized for today's, and tomorrow's, mobile devices. We are
 focused on low-latency input and high frame rates on Android and iOS.
 
+_Flutter is an early-stage open-source project._ We are still missing core
+features like accessibility, text input, localisation, and more. However,
+you can build demos and examples today. We hope you try it out and send
+us [feedback](mailto:flutter-dev@googlegroups.com).
+
  - For information about using Flutter to build apps, please see
    the [getting started guide](http://flutter.io/getting-started/).