Update CHANGELOG

Author: duckontheweb

CHANGELOG.md

@@ -7,6 +7,7 @@
 - License file included in distribution ([#409](https://github.com/stac-utils/pystac/pull/409))
 - Links to Issues, Discussions, and documentation sites ([#409](https://github.com/stac-utils/pystac/pull/409))
 - Python minimum version set to `>=3.6` ([#409](https://github.com/stac-utils/pystac/pull/409))
+- Code of Conduct ([#399](https://github.com/stac-utils/pystac/pull/399))
 
 ### Changed