Added "Barcode Overview"

free-programming-books.md

@@ -321,6 +321,7 @@
 * [97 Things Every Programmer Should Know - Extended](https://leanpub.com/97-Things-Every-Programmer-Should-Know-Extended)
 * [A Mathematical Theory of Communication](http://cm.bell-labs.com/cm/ms/what/shannonday/paper.html) by Claude E.Shannon
 * [Asterisk™: The Definitive Guide](http://asteriskdocs.org/en/3rd_Edition/asterisk-book-html-chunk/index.html)
+* [Barcode Overview](http://www.tec-it.com/download/PDF/Barcode_Reference_EN.pdf)
 * [Come, Let's Play: Scenario-Based Programming Using Live Sequence Charts](http://www.scribd.com/doc/175241338/Come-Let-s-Play)
 * [Communicating Sequential Processes](http://www.usingcsp.com/cspbook.pdf) (PDF) by Tony Hoare
 * [Computer Musings](http://scpd.stanford.edu/knuth/index.jsp) (lectures by Donald Knuth)