Add polymorphic child serializer fix to CHANGELOG.md

Author: josebama

CHANGELOG.md

@@ -20,6 +20,7 @@ any parts of the framework not mentioned in the documentation should generally b
 
 * Ensure that `409 Conflict` is returned when processing a `PATCH` request in which the resource object’s type and id do not match the server’s endpoint properly as outlined in [JSON:API](https://jsonapi.org/format/#crud-updating-responses-409) spec.
 * Properly return parser error when primary data is of invalid type
+* Pass instance to child serializer when `PolymorphicModelSerializer` inits it in `to_internal_value`
 
 ## [3.0.0] - 2019-10-14