Update free-programming-books.md

Added one book on CLP programming in Prolog

free-programming-books.md

@@ -117,6 +117,7 @@
 * [PowerShell](#powershell)
 * [Processing](#processing)
 * [Prolog](#prolog)
+    * [Constraint Logic Programming](#constraint-logic-programming-extended-prolog)
 * [Python](#python)
     * [Django](#django-version)
     * [Flask](#flask)
@@ -1340,6 +1341,10 @@ See also [TeX](#tex)
 * [Warren's Abstract Machine: A Tutorial Reconstruction](http://wambook.sourceforge.net/) - Hassan A¨it-Kaci
 
 
+####Constraint Logic Programming (extended Prolog)
+* [A Gentle Guide to Constraint Logic Programming via ECLiPSe](http://anclp.pl/)
+
+
 ###Python
 * [A Beginner's Python Book](http://en.wikibooks.org/wiki/A_Beginner%27s_Python_Tutorial)
 * [A Bit of Python and Other Things](http://jessenoller.com/good-to-great-python-reads/)