docs: run as systemd service

docs/systemd/wghttp.service

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+[Unit]
+Description=wghttp
+
+[Service]
+Type=simple
+Environment=PRIVATE_KEY=FIXME
+Environment=PEER_KEY=FIXME
+Environment=PEER_ENDPOINT=FIXME
+Environment=CLIENT_IP=FIXME
+ExecStart=%h/go/bin/wghttp --listen 127.0.0.1:1080
+Restart=on-failure
+
+[Install]
+WantedBy=default.target

docs/usage.md

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+# Usage
+
+## Running as systemd service
+
+Since wghttp doesn't need any privilege, it's preferred to run as systemd user service.
+
+Copy [wghttp.service](./systemd/wghttp.service) to `~/.config/systemd/user/wghttp.service`.
+After setting the environment options in `wghttp.service`, run:
+
+```bash
+systemctl --user daemon-reload
+systemctl --user enable --now wghttp
+```