Add /etc/nut reference configuration

Documents the target state for a host that owns a USB UPS and serves it to a
Synology, for hand-configuring a machine where running
setup-nut-synology.sh is not wanted. Verified byte-identical to what that
script generates, so the two cannot drift.

The [admin] password is the only real secret and ships as CHANGEME in both
files that must agree, upsd.users and the MONITOR line of upsmon.conf.

monuser/secret is committed literally. DSM hardcodes both and cannot be
configured otherwise, so they are published constants rather than
credentials, and the upsmon secondary role only permits reading status and
receiving shutdown notifications.

The example UPS serial is a placeholder rather than the real unit's.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nut/upsd.conf.example

@@ -0,0 +1,4 @@
+# Install as /etc/nut/upsd.conf (root:nut, 0640).
+# Must be reachable by the Synology. 0.0.0.0 listens on every interface;
+# narrow it to a specific address if the host is multi-homed.
+LISTEN 0.0.0.0 3493