Add Python version note to README

Author: jakevdp

README.md

@@ -12,6 +12,8 @@ See also the free companion project, [A Whirlwind Tour of Python](https://github
 
 All the code from the book can be found in [code_listings](code_listings).
 The code is in IPython notebooks, organized by book chapter and section.
+All code from this book was tested with Python 3.4-3.5, though it should be
+near 100% compatible with Python 2.7 as well.
 
 
 ## Figure Appendix
@@ -28,4 +30,4 @@ The full notebooks, containing the entire text of the book, will be posted here
 
 ## License
 
-The code here is released under the [MIT license](LICENSE). Read more at the [Open Source Initiative](https://opensource.org/licenses/MIT).
+The code here is released under the [MIT license](LICENSE). Read more at the [Open Source Initiative](https://opensource.org/licenses/MIT).