Created Systemd (markdown)

Systemd.md

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+Run `aquatic-crawler` as `systemd` service.
+
+## System user
+
+To prevent permission issues, it is recommended to run the crawler under the same user as the [aquatic](https://github.com/greatest-ape/aquatic) service. Alternatively, you can change the group permissions and replace the `User` and `Group` below with your own values.
+
+## Service configuration
+
+Create new file `/etc/systemd/system/aquatic-crawler.service` with following settings:
+
+``` snac.service
+[Unit]
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=aquatic
+Group=aquatic
+ExecStartPre=/bin/sleep 10
+ExecStart=/usr/local/bin/aquatic-crawler --infohash=/path/to/ipv6hash.bin --infohash=/path/to/ipv4hash.bin --tracker=udp://host1:port --export-rss=/path/to/rss.xml --export-trackers --enable-tcp
+StandardOutput=file:/home/aquatic/crawler-debug.log
+StandardError=file:/home/aquatic/crawler-error.log
+
+[Install]
+WantedBy=multi-user.target
+```
+* to disable errors output, set `StandardError` as `null`
+
+### Manage
+
+* `sudo systemctl daemon-reload` - apply new configuration
+* `sudo systemctl enable aquatic-crawler` - autostart on system startup
+* `sudo systemctl restart aquatic-crawler` - start the service