Add example qemu startup script, and update documentation

This makes it more convenient to turn on networking on Macs.

Change-Id: I651e0cf05edce042872173bd6114b2db96d58141

docs/qemu.md

@@ -99,7 +99,14 @@ sudo ifconfig tap0 inet6 fc00::/7 up
 
 **NOTE**: One caveat with tuntaposx is that the network interface will
 automatically go down when QEMU exits and closes the network device. So the
-network interface needs to be brought back up each time QEMU is restarted.
+network interface needs to be brought back up each time QEMU is restarted. To
+automate this, you can use the -u flag to run a script on qemu startup. An
+example startup script containing the above command is located in
+scripts/qemu-ifup-macos, so QEMU can be started with:
+
+```
+./scripts/run-magenta-x86-64 -Nu ./scripts/qemu-ifup-macos
+```
 
 ## Debugging the kernel with GDB
 

scripts/qemu-ifup-macos

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+# Copyright 2016 The Fuchsia Authors
+#
+# Use of this source code is governed by a MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT
+
+sudo ifconfig tap0 inet6 fc00::/7 up