update changelog for 1.21.1

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Delta Chat iOS Changelog
 
+## v1.21.1 Testflight
+2021-08
+
+* fix: avoid possible data loss when the app was not closed gracefully before;
+  this bug was introduced in 1.21.1 and not released outside testing groups -
+  thanks to all testers!
+
+
 ## 1.21.0 Testflight
 2021-07