Add link to How To Think Like a Computer Scientist *Interactive edition*

There's a free and pretty great interactive edition of HTLCS. Code samples can be edited and executed in the browser. Some code samples can even be stepped through. Add link to this.

free-programming-books.md

@@ -807,6 +807,7 @@ See also [TeX](#tex)
 * [Hacking Secret Cyphers with Python](http://inventwithpython.com/hacking/chapters/) - Al Sweigart
 * [Hitchiker's Guide to Python!](http://docs.python-guide.org/en/latest/)
 * [How to Think Like a Computer Scientist: Learning with Python](http://www.greenteapress.com/thinkpython/thinkCSpy/)
+	* [How to Think Like a Computer Scientist: Learning with Python, Interactive Edition](http://interactivepython.org/courselib/static/thinkcspy/index.html)
 * [Introduction to Programming Using Python](http://python-ebook.blogspot.com) - Cody Jackson
 * [Invent Your Own Computer Games With Python](http://inventwithpython.com/chapters/) - Al Sweigart
 * [Learn Python The Hard Way](http://learnpythonthehardway.org/book/)