update changelog for 0.4.0 release

Author: Phil Varner

CHANGELOG.md

@@ -108,7 +108,8 @@ Initial release
 - Add links `opportunities` and `create-order` to Product
 - Add link `create-order` to OpportunityCollection
 
-[unreleased]: https://github.com/stapi-spec/stapi-fastapi/compare/v0.3.0...main
+[unreleased]: https://github.com/stapi-spec/stapi-fastapi/compare/v0.4.0...main
+[v0.4.0]: https://github.com/stapi-spec/stapi-fastapi/tree/v0.4.0
 [v0.3.0]: https://github.com/stapi-spec/stapi-fastapi/tree/v0.3.0
 [v0.2.0]: https://github.com/stapi-spec/stapi-fastapi/tree/v0.2.0
 [v0.1.0]: https://github.com/stapi-spec/stapi-fastapi/tree/v0.1.0