add changelog fragment

Author: hmpf

changelog.d/+cleanup-destination-plugins.changed.md

@@ -0,0 +1,6 @@
+Moved a lot of common infrastructure from our NotificationMedium subclasses to
+the parent. This should make it easier to create more media of high quality.
+Also, it should make it easier to use the plugins for validating the
+settings-file elsewhere than just the API.
+
+This might break 3rd party notification plugins.