examples/demo: Adds README

examples/demo/README.md

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+> **WARNING**: This is still highly experimental. This is not usable as a daily
+> driver.
+
+## Building
+
+The stage-2 needs to be built natively on the target architecture (armv7 on
+armv7, aarch64 on aarch64).
+
+(Though the tooling will try to build it through cross-compilation!)
+
+> Note that this has been verified to work on `asus-z00t` on September 24th 2019,
+> using nixpkgs commit `d484f2b7fc0834a068e8ace851faa449a03963f5`.
+
+It should be possible to build both boot images via cross-compilation.
+
+## Burning
+
+To burn the image, build the android-burn-tool, then fastboot it.
+
+```
+nix-build examples/demo/ --argstr device asus-flo -A android-burn-tool
+fastboot boot result
+```
+
+Once booting, it will show a yellow screen, then either a red or a green screen.
+The green screen means that it has found the expected partition to flash. A red
+screen means that the user will need to check what is up.
+
+The command will look like:
+
+```
+dd if=system.img bs=2M status=progress | bin/ssh-initrd dd of=/dev/userdata bs=2M
+```
+
+## Booting
+
+The `boot.img` image can be `fastboot flash`'d into the boot partition, or it
+can be `fastboot boot`ed.
+
+The `boot.img` boot image is expecting to find the system partition using its
+label.
+
+It should also be possible to do this entirely statelessly by burning to an SD
+card, and fastboot booting the device.