Update changelog

Changelog.md

@@ -2,6 +2,7 @@
 Handle Wall, Edge classes in Draw.segment.
 Add an internal PIXI.Point.invert that returns object, for non-keys.
 Fix for test if can convert polygon to rectangle.
+Add `ClipperPaths#union` method that unions polygons without filling.
 
 ## 0.3.3
 Switch to using TextureLoader.getTextureAlphaData.