CHANGELOG-3.4: add "code changes" from 3.3

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

CHANGELOG-3.4.md

@@ -12,6 +12,8 @@ See [code changes](https://github.com/etcd-io/etcd/compare/v3.3.0...v3.4.0) and
 
 **Again, before running upgrades from any previous release, please make sure to read change logs below and [v3.4 upgrade guide](https://github.com/etcd-io/etcd/blob/master/Documentation/upgrades/upgrade_3_4.md).**
 
+- [v3.4.0-rc.0](https://github.com/etcd-io/etcd/releases/tag/v3.4.0-rc.0) (2019-08-12), see [code changes](https://github.com/etcd-io/etcd/compare/v3.3.0...v3.4.0-rc.0).
+
 ### Documentation
 
 - etcd now has a new website! Please visit https://etcd.io.