Update changes

Author: syohex

CHANGES.md

@@ -14,6 +14,9 @@
 *   Improvements:
     - Support drag and drop features on Windows and multiple files' drag and drop
     - Added cmark and cmark-gfm to the markdown command list.
+    - Disable `imenu-submenus-on-top` by default [GH-882][]
+
+  [gh-882]: https://github.com/jrblevin/markdown-mode/issues/882
 
 # Markdown Mode 2.7