README.md in tree_graph-0.2.2 vs README.md in tree_graph-0.2.3

@@ -20,9 +20,11 @@
 
 ## Usage
 
 In Node class, `include TreeGraph`, then implement two methods, `label_for_tree_graph` and `children_for_tree_graph`(which return thing responds to `each`). Then you can call `tree_graph`, `tree_graph_bottom_up` and `tree_graph_bottom_up_in_same_order` on that Node object.
 
+By default, `::Object#label_for_tree_graph` call `to_s`, and `::Object#children_for_tree_graph` return empty array.
+
 Or checkout [test/tree_graph_test.rb](https://github.com/turnon/tree_graph/blob/master/test/tree_graph_test.rb) to see how to use.
 
 Gems depending on tree_graph: [constree](https://github.com/turnon/constree), [trace_tree](https://github.com/turnon/trace_tree).