Add example/example.dart

Update code in README
diff --git a/README.md b/README.md
index 03fa33c..9417820 100644
--- a/README.md
+++ b/README.md
@@ -12,10 +12,13 @@
 import 'package:shelf/shelf_io.dart' as shelf_io;
 import 'package:shelf_proxy/shelf_proxy.dart';
 
-void main() {
-  shelf_io.serve(proxyHandler("https://www.dartlang.org"), 'localhost', 8080)
-      .then((server) {
-    print('Proxying at http://${server.address.host}:${server.port}');
-  });
+void main() async {
+  var server = await shelf_io.serve(
+    proxyHandler("https://dart.dev"),
+    'localhost',
+    8080,
+  );
+
+  print('Proxying at http://${server.address.host}:${server.port}');
 }
 ```
diff --git a/example/example.dart b/example/example.dart
new file mode 100644
index 0000000..bdb4749
--- /dev/null
+++ b/example/example.dart
@@ -0,0 +1,16 @@
+// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:shelf/shelf_io.dart' as shelf_io;
+import 'package:shelf_proxy/shelf_proxy.dart';
+
+void main() async {
+  var server = await shelf_io.serve(
+    proxyHandler("https://dart.dev"),
+    'localhost',
+    8080,
+  );
+
+  print('Proxying at http://${server.address.host}:${server.port}');
+}