docs: Add documentation for the Raspberry Pi 3b

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

README.md

@@ -66,6 +66,9 @@ Currently supported platforms are:
   - [Microsoft Azure](docs/platform-azure.md)
   - [OpenStack](docs/platform-openstack.md)
   - [packet.net](docs/platform-packet.md)
+- Baremetal:
+  - x86 and arm64 servers on [packet.net](docs/platform-packet.md)
+  - [Raspberry Pi Model 3b](docs/platform-rpi3.md)
 
 
 #### Running the Tests

docs/platform-rpi3.md

@@ -0,0 +1,65 @@
+# LinuxKit on a Raspberry Pi 3b
+
+LinuxKit supports building and booting images on a Raspberry Pi 3b
+using the mainline arm64 bit kernels. The LinuxKit arm64 kernel has
+support for some of the devices on the Raspberry Pi 3b, but notably it
+does *not* support:
+
+- WLAN
+- Bluetooth
+- Graphics (though text console should work)
+
+It is unlikely, that we going to add support for these in the main
+LinuxKit kernels in the near future. The LinuxKit kernel is more
+targeted at VMs and baremetal servers where support for these type of
+devices is typically not needed. However, it should be possible to
+easily extend the LinuxKit kernel build process add the required
+kernel options, in a similar fashion to how `-dbg` kernels are
+build. See the [`kernel`](./kernels.md) documentation for details.
+
+
+## Boot
+
+We use the mainline Linux kernels for the Raspberry Pi and it is
+booted via [`uboot`](https://www.denx.de/wiki/U-Boot). The `moby`
+tool, via `moby build -format rpi3 <YAML>`, currently produces a `tar`
+archive which can be extracted onto a FAT32 formatted SD card to boot
+your Raspberry Pi.
+
+Currently, the root filesystem is provided as a RAM disk via the
+`initrd`.
+
+
+## Console
+
+The LinuxKit images support console access via HDMI and USB keyboard
+as well as via serial. For serial console, you need a suitable cable
+to connect to the GPIO pins as described
+[here](https://elinux.org/RPi_Serial_Connection).
+
+
+## Disks
+
+There currently is no support for persistent disks for the Raspberry
+Pi. It may be possible to partition the SD card, format a second
+partition as `ext4` (or similar), and use it for persistent storage.
+
+**TODO:** Experiment with and document this set up.
+
+
+## Networking
+
+The onboard, USB connected network interface is supported by the
+LinuxKit kernel, but for some unknown reason the driver is not cold
+plugged by `mdev`. To use the network interface, the driver needs to
+be `modprobe`d before the network interface can be used. The easiest
+way is to add the following section to the `onboot` section in your
+LinuxKit YAML file:
+
+```
+  - name: netdev
+    image: linuxkit/modprobe:<hash>
+    command: ["modprobe", "smsc95xx"]
+```
+
+**TODO:** Figure out why mdev is not loading the driver.