Add link to behavior tree basic nodes to README

Adding a link to the upstream documentation on behaviortree.dev

Author: Matt Hansen

nav2_behavior_tree/README.md

@@ -74,4 +74,6 @@ The nav2_behavior_tree package provides several navigation-specific nodes that a
 | RateController | Decorator | A node that throttles the tick rate for its child. The tick rate can be supplied to the node as a parameter. The node returns RUNNING when it is not ticking its child. Currently, in the navigation stack, the `RateController` is used to adjust the rate at which the `ComputePathToPose` and `GoalReached` nodes are ticked. |
 | RandomCrawl | Action | This BT action invokes the RandomCrawl ROS2 action server, which is implemented by the nav2_experimental/nav2_rl/nav2_turtlebot3_rl experimental module. The RandomCrawl action server will direct the robot to randomly navigate its environment without hitting any obstacles. |
 | RecoveryNode | Control | The RecoveryNode is a control flow node with two children.  It returns SUCCESS if and only if the first child returns SUCCESS. The second child will be executed only if the first child returns FAILURE. If the second child SUCCEEDS, then the first child will be executed again. The user can specify how many times the recovery actions should be taken before returning FAILURE. In nav2, the RecoveryNode is included in Behavior Trees to implement recovery actions upon failures.
-| Spin | Action | Invokes the Spin ROS2 action server, which is implemented by the nav2_recoveries module. This action is using in nav2 Behavior Trees as a recovery behavior. |
+| Spin | Action | Invokes the Spin ROS2 action server, which is implemented by the nav2_recoveries module. This action is using in nav2 Behavior Trees as a recovery behavior. |
+
+For more information about the behavior tree nodes that are available in the default BehaviorTreeCPP library, see documentation here: https://www.behaviortree.dev/bt_basics/