Add Modelica by Example book (#2268)

free-programming-books.md

@@ -119,6 +119,7 @@
 * [Mercurial](#mercurial)
 * [Mercury](#mercury)
 * [Meta-Lists](#meta-lists)
+* [Modelica](#modelica)
 * [MySQL](#mysql)
 * [Neo4J](#neo4j)
 * [.NET Framework](#net-framework)
@@ -1538,6 +1539,11 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [The Mercury Users' Guide](http://www.mercurylang.org/information/doc-release/user_guide.pdf) (PDF)
 
 
+### Modelica
+
+* [Modelica by Example](http://book.xogeny.com)
+
+
 ### MySQL
 
 * [MySQL Essentials](http://www.techotopia.com/index.php/MySQL_Essentials)