Add README notes on special-skip-keywriter

Author: Tom Dewey

README.adoc

@@ -353,3 +353,14 @@ To re-enable the trigger, remove the override file:
 ----
 $ sudo rm /etc/udev/rules.d/60-rpi-sb-provisioner.rules
 ----
+
+=== Handling pre-signed devices
+
+Pre-signed Raspberry Pi 5 devices cannot execute the key writer without a modification to `rpi-sb-provisioner`, but you can bypass the key writer entirely by using a special file and removing the progress file:
+
+----
+$ sudo rm /var/log/rpi-sb-provisioner/<serial>/progress
+$ sudo touch /var/log/rpi-sb-provisioner/<serial>/special-skip-keywriter
+----
+
+WARNING: This will disable metadata fetching and manufacturing database creation as of writing. This may be resolved in a future version of `rpi-sb-provisioner`.