Merge pull request #1176 from jarcane/master

Added How to Design Programs to Racket section.

free-programming-books.md

@@ -1553,6 +1553,7 @@ See also [TeX](#tex)
 
 
 ### Racket
+* [How to Design Programs 2e](http://www.ccs.neu.edu/home/matthias/HtDP2e/)
 * [Programming Languages: Application and Interpretation](http://cs.brown.edu/courses/cs173/2012/book/index.html)
 * [The Racket Guide](http://docs.racket-lang.org/guide/index.html)