Add Elements of Data Science (#6190)

* Add Element of Data Science

* Fix typo Elements of Data Science

books/free-programming-books-subjects.md

@@ -219,6 +219,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [A Programmer's Guide to Data Mining](http://guidetodatamining.com) - Ron Zacharski (Draft)
 * [Data Jujitsu: The Art of Turning Data into Product](http://www.oreilly.com/data/free/data-jujitsu.csp) (email address *requested*, not required)
 * [Data Mining Algorithms In R](https://en.wikibooks.org/wiki/Data_Mining_Algorithms_In_R) - Wikibooks
+* [Elements of Data Science](https://allendowney.github.io/ElementsOfDataScience/README.html) - Allen B. Downey
 * [Internet Advertising: An Interplay among Advertisers, Online Publishers, Ad Exchanges and Web Users](http://arxiv.org/pdf/1206.1754v2.pdf) (PDF)
 * [Introduction to Data Science](https://docs.google.com/file/d/0B6iefdnF22XQeVZDSkxjZ0Z5VUE/edit?pli=1) - Jeffrey Stanton
 * [Introduction to Data Science](https://leanpub.com/datasciencebook) - Rafael A Irizarry *(Leanpub account or valid email requested)*