Add basic details to the README

Author: kevmoo

webdev/README.md

@@ -1,5 +1,12 @@
 [![Build Status](https://travis-ci.org/dart-lang/webdev.svg?branch=master)](https://travis-ci.org/dart-lang/webdev)
 
+A CLI for Dart web development.
+
+__*Note: Under heavy development.*__
+
+See the [Dart 2 Web Migration Guide](https://webdev.dartlang.org/dart-2) for
+more information.
+
 ## Installation
 
 ```console