Merge pull request #487 from miku/master

Added 'Prolog for Programmers'

free-programming-books.md

@@ -1025,6 +1025,7 @@ See also [TeX](#tex)
 * [Learn Prolog Now!](http://www.learnprolognow.org/)
 * [Logic, Programming and Prolog (2ed)](http://www.ida.liu.se/~ulfni/lpp/)
 * [Natural Language Processing Techniques in Prolog](http://cs.union.edu/~striegnk/courses/nlp-with-prolog/html/)
+* [Prolog for Programmers](https://sites.google.com/site/prologforprogrammers/)
 * [Prolog Techniques](http://bookboon.com/int/student/it/prolog-techniques-applications-of-prolog)
 * [Simply Logical](http://www.cs.bris.ac.uk/~flach/SimplyLogical.html)
 * [Visual Prolog 7.2 for Tyros](http://download.pdc.dk/vip/72/books/tyros/tyros72.pdf) (PDF)