add README.md

related to gokrazy/gokrazy#264

related to gokrazy/gokrazy#263

README.md

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+# gokrazy kernel: Raspberry Pi Linux for the Raspberry Pi
+
+This repository holds a pre-built Linux kernel image for the Raspberry Pi 3, Pi
+4, Pi 5 and Pi Zero 2 W, used by the [gokrazy](https://gokrazy.org/) project.
+
+The files in this repository are picked up automatically by
+the `gok` tool, so you don’t need to interact with this repository
+unless you want to update the kernel to a custom version.
+
+## gokrazy kernel repository map
+
+| repository             | source         | devices                       |
+|------------------------|----------------|-------------------------------|
+| [gokrazy/kernel.rpi]   | [Raspberry Pi] | Pi 3, Pi 4, Pi 5, Pi Zero 2 W |
+| [gokrazy/kernel]       | [kernel.org]   | Pi 3, Pi 4, Pi Zero 2 W       |
+| [gokrazy/kernel.amd64] | [kernel.org]   | PC x86_64, VMs                |
+| [gokrazy/kernel.arm64] | [kernel.org]   | PC arm64, VMs                 |
+
+[Raspberry Pi]: https://github.com/raspberrypi/linux
+[kernel.org]: https://kernel.org/
+[gokrazy/kernel.rpi]: https://github.com/gokrazy/kernel.rpi
+[gokrazy/kernel]: https://github.com/gokrazy/kernel
+[gokrazy/kernel.amd64]: https://github.com/gokrazy/gokrazy/issues/263
+[gokrazy/kernel.arm64]: https://github.com/gokrazy/gokrazy/issues/263
+
+## Cloning the kernel repository
+
+This repository clocks in at over 3 GB of disk usage, so you might want to clone
+it as a shallow clone:
+
+```
+git clone --depth=1 https://github.com/gokrazy/kernel.rpi
+```
+
+## Updating the kernel
+
+First, follow the [gokrazy installation instructions](https://gokrazy.org/quickstart/).
+
+We’re using docker to get a reproducible build environment for our
+kernel images, so install docker if you haven’t already:
+```
+sudo apt install docker.io
+sudo addgroup $USER docker
+newgrp docker
+```
+
+Clone the kernel git repository:
+```
+git clone --depth=1 https://github.com/gokrazy/kernel.rpi
+cd kernel.rpi
+```
+
+Install the kernel-related gokrazy tools:
+```
+go install ./cmd/...
+```
+
+
+And build a new kernel (takes about 5 minutes):
+```
+gokr-rebuild-kernel
+```
+
+The new kernel is stored in the working directory. Use `gok add .` to
+ensure the next `gok` build will pick up your changed files.