docs: note breaking change for constructor of custom plugins

docs/how/updating-datahub.md

@@ -21,6 +21,7 @@ This file documents any backwards-incompatible changes in DataHub and assists pe
 ### Breaking Changes
 
 - #10419 - `aws_session_token` and `aws_region` are now required configurations in the DynamoDB connector. The connector will no longer loop through all AWS regions; instead, it will only use the region passed into the recipe configuration.
+- #10389 - Custom validators, mutators, side-effects dropped a previously required constructor
 
 ### Potential Downtime