Mention comma initializer can be used to concatenate vectors

(inspired by a question on IRC)

doc/snippets/Tutorial_AdvancedInitialization_Join.cpp

@@ -0,0 +1,11 @@
+RowVectorXd vec1(3);
+vec1 << 1, 2, 3;
+std::cout << "vec1 = " << vec1 << std::endl;
+
+RowVectorXd vec2(4);
+vec2 << 1, 4, 9, 16;;
+std::cout << "vec2 = " << vec2 << std::endl;
+
+RowVectorXd joined(7);
+joined << vec1, vec2;
+std::cout << "joined = " << joined << std::endl;