add datacamp cheat sheets (#3727)

free-programming-books.md

@@ -245,6 +245,7 @@
 * [Bitsavers.org](http://bitsavers.trailing-edge.com)
 * [Bookboon: IT & Programming](http://bookboon.com/en/it-programming-ebooks) Cookie required. (email address *requested*, not required)
 * [Cheat Sheets (Free)](https://dzone.com/refcardz)
+* [Cheat Sheets for Data Scientists](https://www.datacamp.com/community/data-science-cheatsheets) - Datacamp (PDF)
 * [conceptf1.blogspot.com](http://conceptf1.blogspot.com/2013/11/list-of-freely-available-programming.html)
 * [Free Smalltalk Books, collected by Stéphane Ducasse](http://stephane.ducasse.free.fr/FreeBooks.html)
 * [Free Tech Books](http://www.freetechbooks.com)