Add version compare links to CHANGELOG

Author: lossyrob

CHANGELOG.md

@@ -270,3 +270,19 @@ use `Band.create`
 ## [v0.3.0] - 2019-10-31
 
 Initial release.
+
+[Unreleased]: <https://github.com/stac-utils/pystac/compare/v0.5.6...main>
+[v0.5.6]: <https://github.com/stac-utils/pystac/compare/v0.5.5..v0.5.6>
+[v0.5.5]: <https://github.com/stac-utils/pystac/compare/v0.5.4..v0.5.5>
+[v0.5.4]: <https://github.com/stac-utils/pystac/compare/v0.5.3..v0.5.4>
+[v0.5.3]: <https://github.com/stac-utils/pystac/compare/v0.5.2...v0.5.3>
+[v0.5.2]: <https://github.com/stac-utils/pystac/compare/v0.5.1...v0.5.2>
+[v0.5.1]: <https://github.com/stac-utils/pystac/compare/v0.5.0...v0.5.1>
+[v0.5.0]: <https://github.com/stac-utils/pystac/compare/v0.4.0...v0.5.0>
+[v0.4.0]: <https://github.com/stac-utils/pystac/compare/v0.3.4...v0.4.0>
+[v0.3.4]: <https://github.com/stac-utils/pystac/compare/v0.3.3...v0.3.4>
+[v0.3.3]: <https://github.com/stac-utils/pystac/compare/v0.3.2...v0.3.3>
+[v0.3.2]: <https://github.com/stac-utils/pystac/compare/v0.3.1...v0.3.2>
+[v0.3.1]: <https://github.com/stac-utils/pystac/compare/v0.3.0...v0.3.1>
+[v0.3.0]: <https://github.com/stac-utils/pystac/tree/v0.3.0>
+