chore(CHANGELOG.md): update changelog with new changes

Author: Hibbins

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Added
+
+- Added support to allow support for manual keys in add_columns as well. Was discussed in: https://github.com/Textualize/textual/discussions/5922
+
 ### Fixed
 
 - Fixed issue with the "transparent" CSS value not being transparent when set using python https://github.com/Textualize/textual/pull/5890