Update Change Log

Author: MarkBaker

CHANGELOG.md

@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Add point size option for scatter charts
 - Basic support for Xlsx reading/writing Chart Sheets [PR #2830](https://github.com/PHPOffice/PhpSpreadsheet/pull/2830)
 
+  Note that a ChartSheet is still only written as a normal Worksheet containing a single chart, not as an actual ChartSheet.
+
 ### Changed
 
 - Memory and speed improvements, particularly for the Cell Collection, and the Writers.