Add suggestion to start state machine in animation tree tutorial (#4652)

tutorials/animation/animation_tree.rst

@@ -261,3 +261,7 @@ Once retrieved, it can be used by calling one of the many functions it offers:
  .. code-tab:: csharp
 
     stateMachine.Travel("SomeState")
+
+The state machine must be running before you can travel. Make sure to either call ``start()`` or choose a node to **Autoplay on Load**.
+
+.. image:: img/animtree18.png