Add CHANGELOG entry

Author: jholveck

CHANGELOG.md

@@ -9,6 +9,7 @@ See Git checking messages for full history.
 - Linux: add the XShmGetImage backend with automatic XGetImage fallback and explicit status reporting (#431)
 - Windows: improve error checking and messages for Win32 API calls (#448)
 - Mac: fix memory leak (#450, #453)
+- improve multithreading: allow multiple threads to use the same MSS object, allow multiple MSS objects to concurrently take screenshots, and document multithreading guarantees (#446, #452)
 - :heart: contributors: @jholveck
 
 ## 10.1.0 (2025-08-16)