Add changelog entry

CHANGELOG.md

@@ -8,6 +8,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Fix extra space on Build sidebar on Firefox !7060
   - Fix HipChat notifications rendering (airatshigapov, eisnerd)
   - Add hover to trash icon in notes !7008 (blackst0ne)
+  - Only show one error message for an invalid email !5905 (lycoperdon)
   - Simpler arguments passed to named_route on toggle_award_url helper method
   - Fix: Backup restore doesn't clear cache
   - Use MergeRequestsClosingIssues cache data on Issue#closed_by_merge_requests method