Update CHANGELOG-EE.md for 9.5.9-ee

[ci skip]

CHANGELOG-EE.md

@@ -61,6 +61,11 @@ Please view this file on the master branch, on stable branches it's out of date.
 - Add group issue boards.
 - Ports style changes fixed in a conflict in ce to ee upstream to master for new projects page.
 
+## 9.5.9 (2017-10-16)
+
+- [SECURITY] Prevent Related Issues from leaking confidential issues.
+- Escape user name in filtered search bar.
+
 ## 9.5.8 (2017-10-04)
 
 - [FIXED] Fix EE delta size check handling with annotated tags.