Update changelog

CHANGELOG.md

@@ -8,6 +8,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added (for new features, dependecies etc.)
 
+* ext_revisions (v0.1 - EXPERIMENTAL)
+    * Creates a backup copy of each entity which is updated via the edit_mode.
+    * Enable via the build property `BUILD_MODULE_EXT_REVISIONS=ENABLED`.
+    * Needs two special entities. A RecordType "Obsolete" and a reference
+      property "revisionOf" with data type "Obsolete".
 * Map (v0.3)
     * Adds a button to the navbar which toggles a map in the top of the main
       panel. The map currently shows all known entities which have geolocation