fix(changelog): add comment on breaking change

- Adds comment on breaking change for opening on focus

Closes #2675

Author: gonzoyumo

Diff for: CHANGELOG.md

@@ -214,6 +214,7 @@ Revert breaking change in **dropdown** ([1a998c4](http://github.com/angular-ui/b
 `*-format` attributes have been renamed to `format-*`
 `min` attribute has been renamed to `min-date`
 `max` attribute has been renamed to `max-date`
+`Open on focus` has been removed. Read more on this ([comment](https://github.com/angular-ui/bootstrap/pull/1922#issuecomment-40491716)). 
 
 - **dropdown:**