Include build documentation

[zozidalom]

📚 Request: Add Build Instructions for NetHunter Kernel

Hi! 👋

First off, thank you for maintaining this project — it's incredibly valuable for the Android modding and NetHunter communities. 🙏

I'm currently trying to build the nethunter branch of this kernel, but I've run into multiple issues, primarily related to:

• Environment variables and expected toolchain setup
• Clang/GCC versions and paths
• Missing or unclear documentation about the build process
• Shell issues like Illegal option -r, likely from a mismatch between bash and sh
• Script or Makefile calls to scripts/-version.sh that don’t exist

It would be extremely helpful if this repository included a short guide or README.md section explaining:

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🔧 Suggested Documentation Topics

• Required build environment (OS, shell, Clang version, Android NDK, etc.)
• Toolchain setup (where to get and how to configure Clang/GCC)
• Kernel config or defconfig to use (e.g. fire_defconfig)
• How to invoke the build process (e.g. via build.sh or make)
• Common pitfalls or known issues (e.g. /bin/sh vs /bin/bash)
• Any device-specific quirks (e.g. for SM-A705 / Galaxy A70)

Even a short note like “Tested on Ubuntu 22.04 + Clang 11 + NDK r23b” would already help a lot. 😊

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🛠️ What I'm Trying to Do

I'm attempting to build the nethunter branch for the Galaxy A70 (SM-A705), and would love to get involved or help test builds — but it's hard to get started without proper docs.

Thanks again for your hard work! If you can point me in the right direction, I’d be happy to contribute some documentation as well.