Add a shelf_web_socket package.

pkg/shelf_web_socket/README.md

+## Web Socket Handler for Shelf
+
+`shelf_web_socket` is a [Shelf][] handler for establishing [WebSocket][]
+connections. It exposes a single function, [webSocketHandler][], which calls an
+`onConnection` callback with a [CompatibleWebSocket][] object for every
+connection that's established.
+
+[Shelf]: pub.dartlang.org/packages/shelf
+
+[WebSocket]: https://tools.ietf.org/html/rfc6455
+
+[webSocketHandler]: https://api.dartlang.org/apidocs/channels/be/dartdoc-viewer/shelf_web_socket/shelf_web_socket.webSocketHandler
+
+[CompatibleWebSocket]: https://api.dartlang.org/apidocs/channels/be/dartdoc-viewer/http_parser/http_parser.CompatibleWebSocket
+
+```dart
+import 'package:shelf/shelf_io.dart' as shelf_io;
+import 'package:shelf_web_socket/shelf_web_socket.dart';
+
+void main() {
+  var handler = webSocketHandler((webSocket) {
+    webSocket.listen((message) {
+      webSocket.add("echo $message");
+    });
+  });
+
+  shelf_io.serve(handler, 'localhost', 8080).then((server) {
+    print('Serving at ws://${server.address.host}:${server.port}');
+  });
+}
+```