add snippets for `src.ui.qtree`

src/README.md

+# `src.ui.maze`
 Below are snippets that show how to define a maze and use the `src.ui.maze` module:
 
 - import the library
@@ -68,3 +69,50 @@ while True:
 - _left click_ on two different maze cells to select them
 - _right click_ to unselect all the cells
 - _ENTER_ to run the path algorithm once the maze generation is done
+
+# `src.ui.qtree`
+Below are snippets that show how to define a maze and use the `src.ui.qtree` module:
+
+- import the library
+```python
+from src.ui.qtree import Canva
+```
+
+- setup the canva
+```python
+canva = Canva(1600, 800, caption='', frame_rate=30)
+canva.setup()
+```
+
+- create a bunch of points
+```python
+from random import random
+
+canva.points = []
+for _ in range(1000):
+    canva.points.append(Point(
+        randint(0, canva.width),
+        randint(0, canva.height),
+        3,
+    ))
+```
+
+- run the animation
+
+```python
+# compute the new set of points, e.g. by moving them a bit randomly
+def update(points: list) -> list:
+    return []
+
+
+# gives back to the canva the things to show.
+# it should be either
+# - a list of booleans, one for each point, that indicates whether or not a
+# point is colliding with others
+# - a quad-tree and the query range
+def f(points: list, mouse: (int, int)):
+    ...
+
+
+canva.loop(update, f)
+```