[readme] formatting

README.md

@@ -3,6 +3,13 @@
 Tutorials for usage of py_trees on ROS2 and, more generally, behaviour trees for
 robotics applications.
 
-Documentation & Walkthrough @ ReadTheDocs: [devel](https://py-trees-ros-tutorials.readthedocs.io/en/devel/), [release-2.0.x](https://py-trees-ros-tutorials.readthedocs.io/en/release-2.0.x/).
+## Documentation
+
+Documentation and tutorials are on ReadTheDocs.
+
+* [devel](https://py-trees-ros-tutorials.readthedocs.io/en/devel/)
+* [release-2.0.x](https://py-trees-ros-tutorials.readthedocs.io/en/release-2.0.x/).
+
+## PyTrees ROS Ecosystem
 
 Refer to [py_trees_ros/README.md](https://github.com/splintered-reality/py_trees_ros/blob/devel/README.md) for more information on the pytrees packages in the ROS ecosystem.