Add changelog entry for autocolorscale fix

Add changelog entry for PR #5439 fixing issue where user-specified
color_continuous_scale was ignored when template had autocolorscale=True.

Author: antonymilne

CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Fixed
+- Fix issue where user-specified `color_continuous_scale` was ignored when template had `autocolorscale=True` [[#5439](https://github.com/plotly/plotly.py/pull/5439)]
+
 ### Updated
 - Speed up `validate_gantt` function [[#5386](https://github.com/plotly/plotly.py/pull/5386)], with thanks to @misrasaurabh1 for the contribution!