add screenshot

Author: fundon

Readme.md

@@ -1,6 +1,8 @@
 # Minimap package
 > A preview of the full source code, likes Sublime Text minimap.
 
+![](https://github.com/fundon/atom-minimap/blob/8f21dad215c444289c4cfea507d14641300f2d9e/resources/minimap-dark.png?raw=true)
+
 ### Todo list:
 
 * ~~Mointor TabPage/File tree state [new, close, move, active]~~