Update changelog

Author: MarkBaker

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Added
 
-- Nothing.
+- CSV Reader - Best Guess for Encoding, and Handle Null-string Escape [#1647](https://github.com/PHPOffice/PhpSpreadsheet/issues/1647)
 
 ### Changed