update changelog

Signed-off-by: Yi Wu <yiwu@pingcap.com>

CHANGELOG.md

@@ -3,6 +3,8 @@ All notable changes to this project are documented in this file.
 See also [TiDB Changelog](https://github.com/pingcap/tidb/blob/master/CHANGELOG.md) and [PD Changelog](https://github.com/pingcap/pd/blob/master/CHANGELOG.md).
 
 ## [Unreleased]
++ Engine
+    - Synchronize all files and directories for received snapshots [#4853](https://github.com/tikv/tikv/pull/4853)
 
 + Raftstore
     - Fix request latency jetter when transferring leader and conf changes. [4734](https://github.com/tikv/tikv/pull/4734)