312: Updated CHANGELOG

Author: jekuaitk

CHANGELOG.md

@@ -11,6 +11,13 @@ before starting to add changes. Use example [placed in the end of the page](#exa
 
 ## [Unreleased]
 
+- [PR-340](https://github.com/OS2Forms/os2forms/pull/340)
+  - Added `OS2FormsHandlerInterface` for reporting misconfigured webform
+  handlers on the webform build and handlers pages.
+  - Implemented configuration validation for the Digital Post handler
+  - Added empty options to the Digital Post handler element selects, so
+  dangling element references no longer display as valid configuration.
+
 ## [5.1.0] 2026-06-03
 
 - [PR-326](https://github.com/OS2Forms/os2forms/pull/326)