added css notes (#3681)

free-programming-books.md

@@ -1425,6 +1425,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Canvassing](https://web.archive.org/web/20160505010319/http://learnjs.io/canvassing/read/)
 * [Code Guide: Standards for developing flexible, durable, and sustainable HTML and CSS](http://mdo.github.io/code-guide/) - Mark Otto
 * [CSS Animation 101](https://github.com/cssanimation/css-animation-101)
+* [CSS Notes for Professionals](http://goalkicker.com/CSSBook) (PDF)
 * [Dive Into HTML5](http://diveinto.html5doctor.com) - Mark Pilgrim ([PDF](http://mislav.net/2011/10/dive-into-html5/))
 * [GA Dash](https://dash.generalassemb.ly)
 * [Google's HTML/CSS Style Guide](https://google.github.io/styleguide/htmlcssguide.html)