Added example for swap on zfs_volume

I could not find an example of how to enable swap on a `zfs_volume` but since I made it work I wanted to share it 😁

example/zfs-encrypted-root.nix

@@ -56,6 +56,24 @@
             options.mountpoint = "/nix";
             mountpoint = "/nix";
           };
+
+          # README MORE: https://wiki.archlinux.org/title/ZFS#Swap_volume
+          "root/swap" = {
+            type = "zfs_volume";
+            size = "10M";
+            content = {
+              type = "swap";
+            };
+            options = {
+              volblocksize = "4096";
+              compression = "zle";
+              logbias = "throughput";
+              sync = "always";
+              primarycache = "metadata";
+              secondarycache = "none";
+              "com.sun:auto-snapshot" = "false";
+            };
+          };
         };
       };
     };

tests/zfs-encrypted-root.nix

@@ -13,5 +13,6 @@ diskoLib.testLib.makeDiskoTest {
   extraTestScript = ''
     machine.succeed("mountpoint /");
     machine.succeed("mountpoint /nix");
+    machine.succeed("swapon --show=NAME | grep /dev/zd"); # i.e. /dev/zd0
   '';
 }