Added latex reference

This is one of my favourite references for latex, since it actually covers a lot of topics, and also references a lot of packages (not only core latex)

free-programming-books.md

@@ -922,6 +922,7 @@
 ###LaTeX
 * [The Not So Short Introduction to LaTeX](http://tobi.oetiker.ch/lshort/lshort.pdf) (PDF)
 * [LaTeX Wikibook](http://en.wikibooks.org/wiki/LaTeX)
+* [Arbirtrary LaTex Reference](http://latex.knobs-dials.com/)
 
 See also [TeX](#tex)