Add launch in Azure Notebooks button (wesm#70)

Adding a link to README.md that makes it easy for users to launch this repository in Azure Notebooks

README.md

@@ -5,6 +5,8 @@ published by O'Reilly Media
 
 [Buy the book on Amazon][1]
 
+<a href="https://notebooks.azure.com/import/gh/wesm/pydata-book"><img src="https://notebooks.azure.com/launch.png" /></a>
+
 # 1st Edition Readers
 
 If you are reading the [1st Edition][1] (published in 2012), please find the
@@ -36,4 +38,4 @@ above, is released under the [MIT license](LICENSE-CODE). Read more at the
 [Open Source Initiative](https://opensource.org/licenses/MIT).
 
 [1]: http://amzn.to/2vvBijB
-[2]: https://github.com/wesm/pydata-book/tree/1st-edition
+[2]: https://github.com/wesm/pydata-book/tree/1st-edition