added haskell link in free-programming-books.md file

free-programming-books.md

@@ -990,6 +990,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [Anatomy of Programming Languages](http://www.cs.utexas.edu/~wcook/anatomy/) - William R. Cook
 * [Beautiful Code, Compelling Evidence](http://vis.renci.org/jeff/2009/01/16/beautiful-code-compelling-evidence/) (PDF)
 * [Exploring Generic Haskell](http://www.andres-loeh.de/ExploringGH.pdf) - Andres Löh
+* [Introduction to Haskell](http://www.seas.upenn.edu/~cis194/spring13/) - Brent Yorgey
 * [Haskell and Yesod](http://www.yesodweb.com/book-1.2)
 * [Haskell no panic](http://lisperati.com/haskell/)
 * [Haskell web Programming](http://yannesposito.com/Scratch/fr/blog/Yesod-tutorial-for-newbies/) (Yesod tutorial)