Make changelog

Author: albertomercurio

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased](https://github.com/qutip/QuantumToolbox.jl/tree/main)
 
 - Return `sesolve` when `mesolve` allows it. ([#455])
+- Simplify structure of `QuantumObjectType`s. ([#456])
 
 ## [v0.30.1]
 Release date: 2025-04-24
@@ -215,3 +216,4 @@ Release date: 2024-11-13
 [#450]: https://github.com/qutip/QuantumToolbox.jl/issues/450
 [#453]: https://github.com/qutip/QuantumToolbox.jl/issues/453
 [#455]: https://github.com/qutip/QuantumToolbox.jl/issues/455
+[#456]: https://github.com/qutip/QuantumToolbox.jl/issues/456