Update changelog

Author: fourjr

CHANGELOG.md

@@ -14,6 +14,7 @@ however, insignificant breaking changes do not guarantee a major version bump, s
 - Upgraded to discord.py v2.0 ([internal changes](https://discordpy.readthedocs.io/en/latest/migrating.html), [GH #2990](https://github.com/kyb3r/modmail/issues/2990)).
 - Python 3.8 or higher is required.
 - Asyncio changes ([gist](https://gist.github.com/Rapptz/6706e1c8f23ac27c98cee4dd985c8120))
+- Plugin registry is purged and all developers have to re-apply due to breaking changes.
 
 ### Added