qemu/arm-versatile: readme: enable ethernet emulation

We build the kernel with smc91x support, so also document how to enable
the emulation of it in qemu.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

board/qemu/arm-versatile/readme.txt

@@ -1,6 +1,6 @@
 Run the emulation with:
 
-  qemu-system-arm -M versatilepb -kernel output/images/zImage -drive file=output/images/rootfs.ext2,if=scsi -append "root=/dev/sda console=ttyAMA0,115200" -serial stdio
+  qemu-system-arm -M versatilepb -kernel output/images/zImage -drive file=output/images/rootfs.ext2,if=scsi -append "root=/dev/sda console=ttyAMA0,115200" -serial stdio -net nic,model=smc91c111 -net user
 
 The login prompt will appear in the terminal that started Qemu. The
 graphical window is the framebuffer.