Update CHANGELOG

Author: duckontheweb

CHANGELOG.md

@@ -7,6 +7,10 @@
 - `ProjectionExtension.crs_string` to provide a single string to describe the coordinate reference system (CRS).
   Useful because projections can be defined by EPSG code, WKT, or projjson.
   ([#548](https://github.com/stac-utils/pystac/pull/548))
+- SAR Extension summaries([#556](https://github.com/stac-utils/pystac/pull/556))
+- Migration for `sar:type` -> `sar:product_type` and `sar:polarization` ->
+  `sar:polarizations` for pre-0.9 catalogs
+  ([#556](https://github.com/stac-utils/pystac/pull/556))
 
 ### Removed