add systemd service

docs/ephemeral.service

+[Unit]
+Description=Ephemeral, An Ephemeral file hosting server
+Documentation=https://git.volkor.me/Volkor/Ephemeral
+After=network.target
+
+[Service]
+Type=simple
+Restart=on-failure
+WorkingDirectory=/opt/ephemeral/
+ExecStart=/opt/ephemeral/target/release/ephemeral
+
+
+[Install]
+WantedBy=multi-user.target
\ No newline at end of file