Add note about displaying material in insecure mode

This was raised on the Juju forum:

https://tinyurl.com/rfwyxwj

Change-Id: Ied8827d1f2ba274d10043adbebee9bff6d172140

src/config.yaml

@@ -66,7 +66,8 @@ options:
     default: false
     description: >-
       FOR TESTING ONLY. Initialise vault after deployment and store the keys
-      locally.
+      locally. Locally stored material can be displayed with:
+          juju run --unit vault/0 leader-get
   auto-generate-root-ca-cert:
     type: boolean
     default: false