add doc

Author: drivebyer

CHANGELOG.md

@@ -10,6 +10,7 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ### Added
 * Add `KeepAfterDelete` in `.Spec.VolumeSpec` to keep pvc after mysql cluster been deleted.
 * Add default resource to init container.
+* Add SidecarImage fields to `.Spec` to allow specifying custom sidecar image.
 
 ### Changed
 ### Removed