Update CHANGELOG

Author: duckontheweb

CHANGELOG.md

@@ -18,6 +18,7 @@
   Objects](https://github.com/stac-extensions/pointcloud#schema-object)
   ([#548](https://github.com/stac-utils/pystac/pull/548))
 - `to_dict` and equality definition for `extensions.item_asset.AssetDefinition` ([#564](https://github.com/stac-utils/pystac/pull/564))
+- `Asset.common_metadata` property ([#563](https://github.com/stac-utils/pystac/pull/563))
 
 ### Removed