add flutter.mdx in configuration/sample-rate

Author: denrase

platform-includes/configuration/sample-rate/flutter.mdx

@@ -0,0 +1,9 @@
+```dart
+import 'package:flutter/widgets.dart';
+import 'package:sentry_flutter/sentry_flutter.dart';
+
+Future<void> main() async {
+  // Capture only 25% of events
+  await SentryFlutter.init((options) => options.sampleRate = 0.25);
+}
+```