Add changelog

Former-commit-id: e6c748e6e697deab5ce38e71c694b3dd8bc1d7be

cpp/CHANGELOG.md

@@ -74,6 +74,7 @@ Please mark all change in change log and use the ticket from JIRA.
 - MS-442 - Merge Knowhere
 - MS-445 - Rename CopyCompleted to LoadCompleted
 - MS-451 - Update server_config.template file, set GPU compute default
+- MS-455 - Distribute tasks by minimal cost in scheduler
 
 ## New Feature
 - MS-343 - Implement ResourceMgr