Added CS book (#4535)

* Update free-programming-books.md

* removed index.html

* Committed suggested changes
Co-authored-by: NSeth <seth@elypia.org>
Co-authored-by: NSeth <seth@elypia.org>

free-programming-books.md

@@ -403,6 +403,7 @@
 * [Computational Thinking](https://www.cs.cmu.edu/~15110-s13/Wing06-ct.pdf) - Jeannette Wing, Carnegie-Mellon University (PDF)
 * [Computer Science I - Draft](https://bitbucket.org/chrisbourke/computersciencei/src) - Dr. Chris Bourke (PDF) (:construction: *in process*)
 * [Computer Science II - Draft](http://cse.unl.edu/~cbourke/ComputerScienceTwo.pdf) - Dr. Chris Bourke (PDF) (:construction: *in process*)
+* [CS Principles: Big Ideas in Programming](https://www.openbookproject.net/books/StudentCSP/) - Mark Guzdial and Barbara Ericson (HTML)
 
 
 #### Computer Vision