Add changelog entry

CHANGELOG.md

@@ -1,8 +1,7 @@
 # NEXT RELEASE
 
 ### Enhancements
-* <New feature description> (PR [#????](https://github.com/realm/realm-core/pull/????))
-* None.
+* Introduce sync 'progress_estimate' parameter (value from 0.0 to 1.0) for existing sync 'ProgressNotifierCallback' api to report sync progress on current batch of upload/download until completion ([#7450](https://github.com/realm/realm-core/issues/7450))
 
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)