Add example for box_transform.

Author: BirjuVachhani

packages/box_transform/example/example.md

@@ -0,0 +1,30 @@
+
+## Resizing a Box
+
+```dart
+final Box rect = Box.fromLTWH(50, 50, 100, 100);
+  
+  final ResizeResult result = BoxTransformer.resize(
+    handle: HandlePosition.bottomRight, // handle that is being dragged
+    initialRect: rect,
+    initialLocalPosition: Vector2.zero(),
+    localPosition: Vector2.zero(),
+    resizeMode: ResizeMode.freeform,
+    initialFlip: Flip.none,
+  );
+  
+  result.rect; // the new rect
+```
+
+## Moving a box.
+
+```dart title="Moving a box"
+  final Box rect = Box.fromLTWH(50, 50, 100, 100);
+  final MoveResult result = BoxTransformer.move(
+    initialRect: rect,
+    initialLocalPosition: Vector2.zero(),
+    localPosition: Vector2.zero(),
+  );
+  
+  result.position; // the new position of the box
+```