Update CHANGELOG with core kubefed API changes
# Unreleased
+- [#887](https://github.com/kubernetes-sigs/kubefed/pull/887) Updates
+ KubefedConfig API based on Kubernetes API conventions.
+- [#885](https://github.com/kubernetes-sigs/kubefed/pull/885) Updates
+ FederatedTypeConfig API based on Kubernetes API conventions.
- [#886](https://sigs.k8s.io/kubefed/issues/886)
The ca bundle for a member cluster is now stored as an optional
field of KubefedCluster since a ca bundle may not be required for
