docs: update release notes for local file embedding feature

Resolves #179

Author: Okeanos

docs/release-notes.md

@@ -18,6 +18,7 @@ nav_order: 9
   flatcar 1.1.0-exp, openshift 4.13.0-exp)_
 - Allow specifying user password hash _(openshift 4.13.0-exp)_
 - Support offline Tang provisioning via pre-shared advertisement _(fcos 1.5.0-exp, openshift 4.13.0-exp)_
+- Support local file embedding for SSH keys and systemd units
 
 ### Bug fixes