README: add instruction for USB ethernet gadget

Signed-off-by: Daniel Maslowski <[email protected]>

README.md

@@ -174,6 +174,22 @@ machine.
 We can use the same trick to cpu to Linux from OSX, but instead of having an Arm tree under `pwd`
 we'll need a Linux binary tree to pick binaries from.
 
+### cpu over USB
+
+There are many IoT like devices that do not have an ethernet port.
+Fear not though: The Linux USB gadget drivers offer ethernet via USB!
+
+There are [tutorials out
+there](https://linuxlink.timesys.com/docs/wiki/engineering/HOWTO_Use_USB_Gadget_Ethernet), and here is the gist:
+
+- enable the Linux kernel options
+  * `CONFIG_USB_GADGET`
+  * `CONFIG_USB_ETH`
+  * `CONFIG_USB_ETH_RNDIS` (for Windows support)
+  * `CONFIG_INET`
+- add the MAC addresses for your gadget device and the machine you connect to in
+  the kernel `CMDLINE`, e.g., `g_ether.dev_addr=12:34:56:78:9a:bc g_ether.host_addr=12:34:56:78:9a:bd`
+
 ## cpu will be familiar to ssh users
 
 As mentioned, cpu looks and feels a lot like ssh, to the point of honoring ssh config files.