add svalbard system as nas; xp11 becomes server

nix/system/svalbard/README.md

@@ -0,0 +1,15 @@
+To set up ZFS on the spinning discs, followed steps derived from these sources (but without encryption):
+* https://ipetkov.dev/blog/installing-nixos-and-zfs-on-my-desktop/
+* https://cheat.readthedocs.io/en/latest/nixos/zfs_install.html
+
+```
+# DISK=<disk from /dev/disk/by-id>
+# POOL=<a name for the pool>
+# zpool create "${POOL}" $DISK
+# zfs create -o compression=on -o mountpoint=legacy "${POOL}/main"
+```
+
+then added the ZFS filesystem to `hardware-configuration.nix` (use the zfs
+created label as the device) and rebooted.
+
+Don't forget to `chown` the mounted system so that non-root can read/write there.