Add ref to Dorogovtsev -- Mendes generator.

src/org/graphstream/algorithm/generator/DorogovtsevMendesGenerator.java

@@ -62,6 +62,9 @@ import java.util.Random;
  * therefore generate trees of any size.
  * </p>
  * 
+ * S. N. Dorogovtsev and J. F. F. Mendes, "Evolution of networks", in Adv. Phys,
+ * 2002, 1079--1187
+ * 
  * @since 20070117
  */
 public class DorogovtsevMendesGenerator extends BaseGenerator {