Make changelog

Author: albertomercurio

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased](https://github.com/qutip/QuantumToolbox.jl/tree/main)
 
+- Change default solver detection in `eigensolve` when using `sigma` keyword argument (shift-inverse algorithm). If the operator is a `SparseMatrixCSC`, the default solver is `UMFPACKFactorization`, otherwise it is automatically chosen by LinearSolve.jl, depending on the type of the operator. ([#580]) 
+
 ## [v0.38.1]
 Release date: 2025-10-27
 
@@ -357,3 +359,4 @@ Release date: 2024-11-13
 [#575]: https://github.com/qutip/QuantumToolbox.jl/issues/575
 [#576]: https://github.com/qutip/QuantumToolbox.jl/issues/576
 [#579]: https://github.com/qutip/QuantumToolbox.jl/issues/579
+[#580]: https://github.com/qutip/QuantumToolbox.jl/issues/580