add missing persist_hostkeys README

Author: sjpb

ansible/roles/persist_hostkeys/README.md

@@ -0,0 +1,8 @@
+# persist_hostkeys
+
+Save hostkeys to persistent storage and restore them after a rebuild/reimage.
+
+Add hosts to the `persist_hostkeys` group to enable.
+
+This role has no variables but hosts in this group must have `appliances_state_dir`
+defined as a directory they can write to on persistent storage.