Document External Push scaler in changelog (#998)

Signed-off-by: Tom Kerkhove <kerkhove.tom@gmail.com>

CHANGELOG.md

@@ -21,7 +21,8 @@
 - Provide KEDA go-client ([#494](https://github.com/kedacore/keda/issues/494))
 - Define KEDA readiness and liveness probes ([#788](https://github.com/kedacore/keda/issues/788))
 - KEDA Support for configurable scaling behavior in HPA v2beta2 ([#802](https://github.com/kedacore/keda/issues/802))
-- Adding Standard Resource metrics to KEDA ([#874](https://github.com/kedacore/keda/pull/874))
+- Add External Push scaler ([#820](https://github.com/kedacore/keda/issues/820) | [docs](https://keda.sh/docs/2.0/scalers/external-push/))
+- Add Standard Resource metrics to KEDA ([#874](https://github.com/kedacore/keda/pull/874))
 - Managed Identity support for Azure Monitor scaler ([#936](https://github.com/kedacore/keda/issues/936))
 - Add support for multiple triggers on ScaledObject ([#476](https://github.com/kedacore/keda/issues/476))
 - Add consumer offset reset policy option to Kafka scaler ([#925](https://github.com/kedacore/keda/pull/925))