add screenshot to readme

Author: Green-Sky

README.md

@@ -3,6 +3,8 @@ A drop-in, single-file entity editor for EnTT, with ImGui as graphical backend.
 
 [demo-code](https://github.com/Green-Sky/imgui_entt_entity_editor_demo) [(live)](http://scam.rocks/imgui_entt_entity_editor_demo/)
 
+![screenshot](https://github.com/Green-Sky/imgui_entt_entity_editor_demo/blob/master/imgui_entt_entity_editor_screenshot0.png)
+
 # example usage
 ```c++
 struct Transform {