update CHANGELOG.md (#556)

followup for #549 

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Updated Unreleased notes to document a new helper that adds caching
support for streamed components.
* Confirmed the previously noted breaking change around prerender
caching remains unchanged.
* Clarified this is a docs-only update with no runtime behavior or
public API changes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: ihabadham

CHANGELOG.md

@@ -16,6 +16,9 @@ You can find the **package** version numbers from this repo's tags and below in
 ## [Unreleased]
 *Add changes in master not yet tagged.*
 
+### Added
+- Added `cached_stream_react_component` helper method, similar to `cached_react_component` but for streamed components.
+
 ### Changed (Breaking)
 - `config.prerender_caching`, which controls caching for non-streaming components, now also controls caching for streamed components. To disable caching for an individual render, pass `internal_option(:skip_prerender_cache)`.