chore: update CHANGELOG with recent changes

CHANGELOG.md

@@ -5,6 +5,14 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 This file follows the convention described at
 [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
+## [Unreleased]
+### Changed
+- **Breaking**: 5ohue: added `CompositeOperator` in place of `bindings::CompositeOperator`.
+  Instead of using `bindings::CompositeOperator_LightenIntensityCompositeOp` one must now
+  use `CompositeOperator::LightenIntensity`.
+### Added
+- 5ohue: added `sigmoidal_contrast_image()` function to the wand API.
+
 ## [0.19.1] - 2023-09-23
 ### Changed
 - Feature `disable-hdri` is now enabled by default to work around an apparent