Add Unreleased section

Author: albertomercurio

CHANGELOG.md

@@ -5,6 +5,8 @@ All notable changes to [`QuantumToolbox.jl`](https://github.com/qutip/QuantumToo
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
 ## [v0.22.0] (2024-11-20)
 
 - Change the parameters structure of `sesolve`, `mesolve` and `mcsolve` functions to possibly support automatic differentiation. ([#311])