Added "Commentary on the Sixth Edition UNIX Operating System"

free-programming-books.md

@@ -1476,6 +1476,7 @@ See also [LaTeX](#latex)
 
 ###Unix
 * [A User's Guide for GNU AWK](http://www.math.utah.edu/docs/info/gawk_toc.html)
+* [Commentary on the Sixth Edition UNIX Operating System](http://www.lemis.com/grog/Documentation/Lions/) - J. Lions
 * [Conquering the Command Line](http://www.conqueringthecommandline.com/)
 * [Unix Toolbox](http://cb.vu/unixtoolbox.xhtml) - Colin Barschel