docs: add PREEMPT_RT kernel build guide for KOS image

[Tanay-dev]

This PR introduces README.md, a step-by-step guide for cross-compiling
and deploying a full PREEMPT_RT-enabled Linux 6.12.x kernel directly into the KOS
SD-card image, using Ubuntu 24.04 LTS.

Key sections:

• Set up cross-compile toolchain on Ubuntu
• Clone and verify the rpi-6.12.y branch of the Raspberry Pi kernel tree
• Enable full PREEMPT_RT and High-Res timers (interactive and scripted)
• Fast incremental builds (make Image modules dtbs)
• Mounting KOS image partitions (/mnt/kos-boot, /mnt/kos-root)
• Installing modules, copying Image, DTBs and overlays
• Updating config.txt for KOS
• Verifying with strings … | grep "Linux version" and on-device uname -a
• Optional QEMU chroot dry-run for sanity checks

All commands match those tested on my Ubuntu host and reflect the exact steps I used for my KOS build.