Merge pull request #1017 from yaph/exploreflask

Added Explore Flask by Robert Picard

free-programming-books.md

@@ -1431,6 +1431,7 @@ See also [TeX](#tex)
 
 
 ####Flask
+* [Explore Flask](http://exploreflask.com/) - Robert Picard
 * [The Flask Mega-Tutorial](http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world) - Miguel Grinberg (0.9)