Mention search improvements in CHANGELOG

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

@@ -43,6 +43,7 @@ v 7.13.0 (unreleased)
   - Redesign project page. Show README as default instead of activity. Move project activity to separate page
   - Make left menu more hierarchical and less contextual by adding back item at top
   - A fork can’t have a visibility level that is greater than the original project.
+  - Faster code search in repository and wiki. Fixes search page timeout for big repositories 
 
 v 7.12.2
   - Correctly show anonymous authorized applications under Profile > Applications.