added Coding Guidelines for Prolog

free-programming-books.md

@@ -1522,6 +1522,7 @@ See also [TeX](#tex)
 * [Applications of Prolog](http://bookboon.com/int/student/it/applications-of-prolog)
 * [Building Expert Systems in Prolog](http://www.amzi.com/ExpertSystemsInProlog/)
 * [Concise Intro to Prolog](http://www.cis.upenn.edu/~matuszek/Concise%20Guides/Concise%20Prolog.html)
+* [Coding Guidelines for Prolog](http://arxiv.org/abs/0911.2899) - Michael A. Covington, Roberto Bagnara, Richard A. O'Keefe, Jan Wielemaker, Simon Price
 * [GNU Prolog Manual](http://www.gprolog.org/manual/gprolog.pdf) (PDF)
 * [Introduction to Prolog for Mathematicians](http://www.j-paine.org/prolog/mathnotes/files/pms/pms.html)
 * [Learn Prolog Now!](http://www.learnprolognow.org/)