Add "Notes on Programming in TeX"

free-programming-books.md

@@ -956,6 +956,7 @@ See also [TeX](#tex)
 
 ###TeX
 * [TeX for the Impatient](https://www.gnu.org/software/teximpatient/), by Paul Abrahams, Kathryn Hargreaves, and Karl Berry
+* [Notes On Programming in TeX](http://pgfplots.sourceforge.net/TeX-programming-notes.pdf) (PDF) by Christian Feursänger
 
 See also [LaTeX](#latex)