Update CHANGELOG.md

Author: zihejia

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## [v2.1.0](https://github.com/mixpanel/mixpanel-flutter/tree/v2.1.0) (2023-03-06)
 
+### NOTE:
+- From this version we will prefix randomly generated device-specific distinct_ids with "$device:". The prefix is applied the next time a new random ID is generated, any IDs generated by previous SDK versions and  persisted on the device will continue to be used as-is until reset is called to generate a new ID. This does not change the value sent for the $device_id property, which will continue to be the randomly-generated ID without a prefix. Mixpanel's $identify endpoint has been updated to accept UUIDs with this prefix to coordinate with this change.
+
 ### Enhancements
 
 - bump ios to 4.0.5 and android to 7.3.0 [\#99](https://github.com/mixpanel/mixpanel-flutter/pull/99)