doc: add image to readme

close justinmk#195
close justinmk#90

Author: justinmk

README.md

@@ -19,6 +19,8 @@ Features
 - _Powerful:_ `:Shdo[!]` generates shell script
 - _Reliable:_ Less code, fewer bugs (96% smaller than netrw). Supports Vim 7.2+
 
+![](./dirvish.png)
+
 Concepts
 --------