Add fixed issues to changelog
# Unreleased
+- [#720](https://github.com/kubernetes-sigs/federation-v2/issues/720) -
+ `kubefed2 enable` now succeeds if federation of the type is already
+ enabled.
+ - [#738](https://github.com/kubernetes-sigs/federation-v2/issues/738) -
+ Cleanup `kubefed2 enable` required arguments and remove unnecessary
+ `--registry-namespace` option from `kubefed2 <enable|disable>`.
- [#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
