CHANGELOG: add notes on breaking change for SelfHosted
## [Unreleased 0.3.1]
+**Notes for self-hosted etcd**:
+The newly introduced TLS feature for self hosted etcd is a breaking change.
+Existing self hosted etcd cluster MUST be recreated for updating to this release.
+
### Added
- Self-hosted etcd: if `etcd-hosts.checkpoint` file exists under `${datadir}/`,
