add cover to readme because pretty

Author: schacon

README.md

@@ -6,6 +6,8 @@ You can find this book online (soon) at: http://atom.io/docs
 
 This book is open source under a Creative Commons license.
 
+![atom cover](https://cloud.githubusercontent.com/assets/70/5341501/de057c62-7ef6-11e4-8926-42b6802d6d38.png)
+
 ## Current Progress
 
 To check the current progress and planned content of the book, check out the [outline](outline.md).