[orx-quadtree] add quadtree module (#174)

orx-quadtree/README.md

@@ -0,0 +1,22 @@
+# orx-quadtree
+
+An extension for creating a [Quadtree](https://en.wikipedia.org/wiki/Quadtree) for points. A quadtree is a spatial
+partioning tree structure meant to provide fast spatial queries such as nearest points within a range.
+
+## Example
+
+```kotlin
+val box = Rectangle.fromCenter(Vector2(400.0), 750.0)
+
+val quadTree = Quadtree<Vector2>(box) { it }
+
+for (point in points) {
+    quadTree.insert(point)
+}
+
+val nearestQuery = quadTree.nearest(points[4], 20.0)
+```
+
+### Author
+
+Ricardo Matias / [@ricardomatias](https://github.com/ricardomatias)