contrib/systemd: add systemd-networkd examples from manuals

Change-Id: I265637f39dd16dd43992f33149e512e34ed83252
diff --git a/contrib/systemd/apn0.netdev b/contrib/systemd/apn0.netdev
new file mode 100644
index 0000000..b8db5e1
--- /dev/null
+++ b/contrib/systemd/apn0.netdev
@@ -0,0 +1,7 @@
+[NetDev]
+Name=apn0
+Kind=tun
+
+[Tun]
+User=username
+Group=username