add integration tests and update README

example/test_driver/integration_test_update_goldens.dart

@@ -0,0 +1,22 @@
+import 'dart:io';
+import 'package:crypto/crypto.dart';
+import 'package:integration_test/integration_test_driver_extended.dart';
+
+Future<void> main() async {
+  await integrationDriver(
+    onScreenshot: (
+      String screenshotName,
+      List<int> screenshotBytes, [
+      Map<String, Object?>? args,
+    ]) async {
+      final dir = screenshotName.split("/")[0];
+      final name = screenshotName.split("/")[1];
+      final File image = await File('integration_test/goldens/$dir/$name.png')
+          .create(recursive: true);
+
+      image.writeAsBytesSync(screenshotBytes);
+
+      return true;
+    },
+  );
+}