docs: Update changelog (#3364)

Author: fritzchr

axum/CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+- **added:** Implement `OptionalFromRequest` for `Multipart` ([#3220])
 - **changed:** `serve` has an additional generic argument and can now work with any response body
   type, not just `axum::body::Body` ([#3205])