Update CHANGELOG

CHANGELOG.md

@@ -3,6 +3,12 @@
 The present file will list all changes made to the project; according to the
 [Keep a Changelog](http://keepachangelog.com/) project.
 
+## [9.5.4] unreleased
+
+### Changed
+
+- `iframe` elements are not anymore allowed in rich text unless `GLPI_ALLOW_IFRAME_IN_RICH_TEXT` constant is defined to `true`
+
 ## [9.5.3] 2020-11-25
 
 ### Deprecated