Added "Prolog Problems"

free-programming-books.md

@@ -1203,6 +1203,7 @@ See also [TeX](#tex)
 * [Prolog and Natural-Language Analysis](http://www.mtome.com/Publications/PNLA/pnla-digital.html) - Fernando C. N. Pereira, Stuart M. Shieber
 * [Prolog Experiments in Discrete Mathematics, Logic, and Computability](http://web.cecs.pdx.edu/~jhein/books/PrologLabBook09.pdf) - James L. Hein
 * [Prolog for Programmers](https://sites.google.com/site/prologforprogrammers/)
+* [Prolog Problems](https://sites.google.com/site/prologsite/prolog-problems) - Werner Hett
 * [Prolog Programming: A First Course](http://engronline.ee.memphis.edu/deductiveDB/prolog_book.pdf) - Paul Brna
 * [Prolog Techniques](http://bookboon.com/int/student/it/prolog-techniques-applications-of-prolog)
 * [Simply Logical](http://www.cs.bris.ac.uk/~flach/SimplyLogical.html)