add MongoDB scaler to Changelog
- Add Redis cluster support for Redis list and Redis streams scalers ([#1437](https://github.com/kedacore/keda/pull/1437))
- Global authentication credentials can be managed using `ClusterTriggerAuthentication` objects ([#1452](https://github.com/kedacore/keda/pull/1452))
- Introducing OpenStack Swift scaler ([#1342](https://github.com/kedacore/keda/issues/1342))
+- Introducing MongoDB scaler ([#1467](https://github.com/kedacore/keda/pull/1467))
### Improvements
