contrib: add example systemd unit file

contrib/README.md

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+## contrib
+
+Scripts and files which may be useful but aren't part of the core etcd project.
+
+- [systemd](systemd) - an example unit file for deploying etcd on systemd-based distributions

contrib/systemd/etcd.service

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+[Unit]
+Description=etcd key-value store
+Documentation=https://github.com/coreos/etcd
+
+[Service]
+User=etcd
+Type=notify
+Environment=ETCD_DATA_DIR=/var/lib/etcd
+Environment=ETCD_NAME=%m
+ExecStart=/usr/bin/etcd
+Restart=always
+RestartSec=10s
+LimitNOFILE=40000
+
+[Install]
+WantedBy=multi-user.target