Update CHANGELOG.md for 8.15.6

[ci skip]

CHANGELOG.md

@@ -181,6 +181,13 @@ entry.
 - Add margin to markdown math blocks.
 - Add hover state to MR comment reply button.
 
+## 8.15.6 (2017-02-14)
+
+- Patch Asciidocs rendering to block XSS.
+- Fix XSS vulnerability in SVG attachments.
+- Prevent the GitHub importer from assigning labels and comments to merge requests or issues belonging to other projects.
+- Patch XSS vulnerability in RDOC support.
+
 ## 8.15.5 (2017-01-20)
 
 - Ensure export files are removed after a namespace is deleted.