Add preview image

Author: nekocode

README.md

@@ -1,14 +1,3 @@
 # drag_down_to_pop
 
-A new Flutter package project.
-
-## Getting Started
-
-This project is a starting point for a Dart
-[package](https://flutter.dev/developing-packages/),
-a library module containing code that can be shared easily across
-multiple Flutter or Dart projects.
-
-For help getting started with Flutter, view our 
-[online documentation](https://flutter.dev/docs), which offers tutorials, 
-samples, guidance on mobile development, and a full API reference.
+<kbd><img border="1" src="https://github.com/nekocode/drag-down-to-pop-flutter/blob/master/image/preview.gif?raw=true"></img></kbd>