README: start adding instructions

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 Crux-arm RaspberryPi2 Linux Kernel
 
+NOTE: This README is wip. Some instructions are either vague or missing.
+
 upstream sources:
   https://www.kernel.org/pub/linux/kernel/v4.x
   git://git.kernel.org/pub/scm/linux/kernel/git/statble/linux-stable.git
   https://github.com/raspberrypi/linux
 
+Type make or make all to build kernel image, the device tree files/overlays,
+modules and related files. You may also find some of the other targets useful;
+either for rebuilding only a certain target or possibly downloading files for
+later offline build. These currently include:
+
+all:        build all targets
+clean:      clean out built files/directories
+distclean:  clean, plus remove even downloaded files also
+files:      download all remote files
+config:     unpatch the kernel and copy the default config into the kernel tree
+image:      build zImage and rpi2 prep'd kernel image files
+dtb:        build device tree database/overlay files, tarball
+modules:    build kernel modules, tarball
+menuconfig: run menuconfig for kernel, starting with default kernel config
+
+For booting the kernel, you will also need to download these addition files:
+bootcode.bin: https://github.com/raspberrypi/firmware/raw/master/boot/bootcode.bin
+start.elf: https://github.com/raspberrypi/firmware/raw/master/boot/start.elf
+
+Sd card layout:
+
+first partition, boot (fat32):
+kernel-$version.img, System.map-$version, bootcode.bin, start.elf, config.txt, cmdline.txt
+Untar the dtb tarball (dtb-$version.tar.*) to the boot partition.
+
+second partition, root (ext{2,3,4}, xfs, btrfs, etc):
+Untar the modules tarball (modules-$version.tar.*) to the root partition.
+Untar all the package files created by crossrootfs and raspberrypi2-arm to
+the root partition.
 
 Generating the kernel patch: