add new include_plotlyjs='cdn'behaviour to changelog

Author: adehad

CHANGELOG.md

@@ -7,6 +7,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 
 ### Fixed
+- Plotly.js cdn url will now be versioned by default for:
+`include_plotlyjs='cdn'` a new `include_plotlyjs='cdn-latest'` option
+has the original behaviour. Prevents likelihood of htmls generated with older `plotly.js` versions breaking with version bumps. [2961](https://github.com/plotly/plotly.py/pull/2961)
 
 ### Updated