Merge pull request jeroenjanssens#54 from nchase/patch-1

Link book image to O'Reilly Store Page

Author: jeroenjanssens

README.md

@@ -1,7 +1,7 @@
 Data Science at the Command Line
 ================================
 
-![Data Science at the Command Line Cover](book/.cover.png)
+[![Data Science at the Command Line Cover](book/.cover.png)](http://shop.oreilly.com/product/0636920032823.do)
 
 This repository contains the virtual machine, data, scripts, and custom command-line tools used in the book [Data Science at the Command Line](http://datascienceatthecommandline.com).
 ## License