[orx-hash-grid] Add documentation, demos and improve API

orx-hash-grid/README.md

@@ -1,3 +1,29 @@
 # orx-hash-grid
 
-A 2D space partitioning for points.
+A 2D space partitioning for points.
+
+## Usage
+
+Create a hash grid for a given radius. 
+```kotlin
+val grid = HashGrid(radius)
+```
+
+Check for a given query point if the grid is free, i.e. there is no point in the grid at distance less than `radius` away from the
+query point.
+
+```kotlin
+grid.isFree(query)
+```
+
+Add a point to the hash grid structure: 
+```kotlin
+grid.insert(point)
+```
+
+Iterate over all points in the hash grid:
+```kotlin 
+for (point in grid.points()) {
+    // do something with point
+}
+```