Update CHANGELOG-EE.md for 11.2.4-ee

[ci skip]

CHANGELOG-EE.md

@@ -76,6 +76,14 @@ Please view this file on the master branch, on stable branches it's out of date.
 - Remove differences between CE and EE settings panel component.
 
 
+## 11.2.4 (2018-09-26)
+
+### Security (2 changes)
+
+- Project groups approvers no longer leak private groups info.
+- Protect against CSRF attacks when adding Slack app.
+
+
 ## 11.2.3 (2018-08-28)
 
 - No changes.