update changelog for FederationConfig resource usage
# Unreleased
+ - [#737](https://github.com/kubernetes-sigs/federation-v2/pull/737) -
+ Switch to use FederationConfig resource rather than command line
+ options for federation controller configuration management
- [#549](https://github.com/kubernetes-sigs/federation-v2/pull/549) -
As a result of watching only labled resources, unlabled resources
in unselected clusters will no longer be deleted.
