add sample systemd files

contrib/systemd/cubby.user.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Cubby in-memory key-value cache (per-user)
+Documentation=https://git.juancwu.dev/juancwu/cubby
+After=default.target
+
+[Service]
+Type=simple
+# Path to the cubby binary. Build with:
+#   go build -o ~/.local/bin/cubby ./cmd/cubby
+ExecStart=%h/.local/bin/cubby -socket %t/cubby.sock
+# %t expands to $XDG_RUNTIME_DIR (e.g. /run/user/1000), which is private to
+# the user, so the default 0600 socket permissions are fine.
+
+Restart=on-failure
+RestartSec=2s
+
+[Install]
+WantedBy=default.target