Provide an example systemd service file (#77)

* Create README.md

* Create wireproxy.service

* Update README.md

Author: villepeh

systemd/README.md

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+# Running wireproxy with systemd
+
+If you're on a systemd-based distro, you'll most likely want to run Wireproxy as a systemd unit.
+
+The provided systemd unit assumes you have the wireproxy executable installed on `/opt/wireproxy/wireproxy` and a configuration file stored at `/etc/wireproxy.conf`. These paths can be customized by editing the unit file.
+
+# Setting up the unit
+
+1. Copy the `wireproxy.service` file from this directory to `/etc/systemd/system/`, or use the following cURL command to download it:
+   ```bash
+   sudo curl https://raw.githubusercontent.com/pufferffish/wireproxy/master/systemd/wireproxy.service > /etc/systemd/system/wireproxy.service
+   ```
+
+2. If necessary, customize the unit.
+
+   Edit the parts with `ExecStartPre=` and `ExecStart=` to point to the executable and the configuration file. For example, if wireproxy is installed on `/usr/bin` and the configuration file is located in `/opt/myfiles/wireproxy.conf` do the following change:
+   ```service
+   ExecStartPre=/usr/bin/wireproxy -n -c /opt/myfiles/wireproxy.conf
+   ExecStart=/usr/bin/wireproxy -c /opt/myfiles/wireproxy.conf
+   ```
+   #### 2.2 Drop root privileges (optional, but recommended)
+   Without any modifications, this Wireproxy service will run as root. You might want to drop those privileges. One way to do this is to simply create a system account for Wireproxy (or just use your own user account to run it instead).
+   ```bash
+   sudo useradd --comment "Wireproxy tunnel" --system wireproxy
+   ```
+   Then uncomment these lines from the wireproxy.service:
+   ```service
+   #User=wireproxy
+   #Group=wireproxy
+   ```
+   Caveats:
+     1) Make sure `wireproxy` user can read the wireproxy configuration file.
+     2) Also note that unprivileged user cannot bind to ports below 1024 by default.
+
+4. Reload systemd and enable the unit.
+   ```bash
+   sudo systemctl daemon-reload
+   sudo systemctl enable --now wireproxy.service
+   ```
+
+5. Make sure it's working correctly.
+
+   Finally, check out the unit status to confirm `wireproxy.service` has started without problems. You can use commands like `systemctl status wireproxy.service` and/or `sudo journalctl -u wireproxy.service`.
+
+# Additional notes
+
+If you want to disable the extensive logging that's done by Wireproxy, simply add `-s` parameter to `ExecStart=`. This will enable the silent mode that was implemented with [pull/67](https://github.com/pufferffish/wireproxy/pull/67).

systemd/wireproxy.service

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+[Unit]
+Description=Wireproxy socks5/http tunnel
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+#Uncomment and/or change these if you don't want to run Wireproxy as root
+#User=wireproxy
+#Group=wireproxy
+Type=simple
+Restart=on-failure
+RestartSec=30s
+ExecStartPre=/opt/wireproxy/wireproxy -n -c /etc/wireproxy.conf
+ExecStart=/opt/wireproxy/wireproxy -c /etc/wireproxy.conf
+SyslogIdentifier=wireproxy
+
+[Install]
+WantedBy=multi-user.target