Add a top-level status library.

README.md

 The `web_socket_channel` package provides [`StreamChannel`][stream_channel]
 wrappers for WebSocket connections. It provides a cross-platform
 [`WebSocketChannel`][WebSocketChannel] API, a cross-platform implementation of
 that API that communicates over an underlying [`StreamChannel`][stream_channel],
 [an implementation][IOWebSocketChannel] that wraps `dart:io`'s `WebSocket`
 class, and [a similar implementation][HtmlWebSocketChannel] that wrap's
 `dart:html`'s.
 
 [stream_channel]: https://pub.dartlang.org/packages/stream_channel
 [WebSocketChannel]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel-class.html
 [IOWebSocketChannel]: https://www.dartdocs.org/documentation/web_socket_channel/latest/io/IOWebSocketChannel-class.html
 [HtmlWebSocketChannel]: https://www.dartdocs.org/documentation/web_socket_channel/latest/html/HtmlWebSocketChannel-class.html
 
+It also provides constants for the WebSocket protocol's pre-defined status codes
+in the [`status.dart` library][status]. It's strongly recommended that users
+import this library should be imported with the prefix `status`.
+
+[status]: https://www.dartdocs.org/documentation/web_socket_channel/latest/status/status-library.html
+
+```dart
+import 'package:web_socket_channel/io.dart';
+import 'package:web_socket_channel/status.dart' as status;
+
+main() async {
+  var channel = await IOWebSocketChannel.connect("ws://localhost:1234");
+
+  channel.stream.listen((message) {
+    channel.sink.add("received!");
+    channel.close(status.goingAway);
+  });
+}
+```
+
 ## `WebSocketChannel`
 
 The [`WebSocketChannel`][WebSocketChannel] class's most important role is as the
 interface for WebSocket stream channels across all implementations and all
 platforms. In addition to the base `StreamChannel` interface, it adds a
 [`protocol`][protocol] getter that returns the negotiated protocol for the
 socket, as well as [`closeCode`][closeCode] and [`closeReason`][closeReason]
 getters that provide information about why the socket closed.
 
 [protocol]: https://www.dartdocs.org/documentation/web_socket_channel/latest/web_socket_channel/WebSocketChannel/protocol.html
@@ skipping 74 matching lines @@
 import 'package:web_socket_channel/html.dart';
 
 main() async {
   var channel = new HtmlWebSocketChannel.connect("ws://localhost:8181");
   channel.sink.add("connected!");
   channel.sink.listen((message) {
     // ...
   });
 }
 ```