Update CHANGELOG

Author: duckontheweb

CHANGELOG.md

@@ -11,6 +11,7 @@
 - `ItemCollection` class for working with GeoJSON FeatureCollections containing only
   STAC Items ([#430](https://github.com/stac-utils/pystac/pull/430))
 - Support for Python 3.9 ([#420](https://github.com/stac-utils/pystac/pull/420))
+- Migration for pre-1.0.0-rc.1 Stats Objects (renamed to Range Objects in 1.0.0-rc.3) ([#447](https://github.com/stac-utils/pystac/pull/447))
 
 ### Changed