fix: an upgrade keeps the configuration and asks nothing.

README.md

@@ -85,8 +85,10 @@ that deploy a router on Azure or GCP.
 ## Configuration
 
 `/etc/vpn-router/vpn-router.json` is JSON, mode 0600, and is created on first install. It
-is managed with `ucf`, so it is yours to edit: a later `dpkg-reconfigure` applies what
-changed and asks before touching anything you altered by hand. A complete example is
+is managed with `ucf`, so it is yours to edit: a later `dpkg-reconfigure` applies the answers
+given and asks before touching anything you altered by hand, and an upgrade leaves the file as
+it is and asks nothing. Settings the file leaves out take their defaults when it is read, so a
+file written by cloud-init or by hand needs only the settings that differ. A complete example is
 installed at `/usr/share/doc/vpn-router/vpn-router.json.example`.
 
 The file is an object of sections, each an object of settings. A section or setting that