Update README.md

Author: mackysoft

README.md

@@ -6,6 +6,8 @@ The `SerializeReference` attribute, added in Unity 2019.3, makes it possible to
 
 The `SubclassSelector` attribute allows you to easily set subclasses of those abstract classes in the Editor that are serialized by `SerializeReference` attribute.
 
+![SubclassSelector](https://user-images.githubusercontent.com/13536348/113597922-5d912500-9677-11eb-8faa-a33dabc60aa3.gif)
+
 #### Features
 
 - Easily set subclass by popup.