Update README.md

README.md

@@ -30,6 +30,7 @@ Check the here the [TODO list](TODO).
 # Tutorials
 
 - [Install and test](install_and_test.md)
+- [Google collab: use the octree with multiple particles per leaf](https://colab.research.google.com/drive/1EObmEt7XK56EpwHh7PQ-viR7PHZ5H0pl)
 - [A refined PM DMO run with fixed timesteps](run_pm_dmo_NFW_fixed_timestep.md)
 - **soon:** A cosmological PM DMO run with fixed timesteps