Add detailed readme

Author: kdrag0n

README.md

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+# Proton Clang
+
+This is a [LLVM](https://llvm.org/) and [Clang](https://clang.llvm.org/) compiler toolchain built for kernel development. Builds are always made from the latest LLVM sources rather than stable releases, so complete stability cannot be guaranteed.
+
+This toolchain targets the AArch32, AArch64, and x86 architectures. It is built with LTO and PGO to reduce compile times as much as possible. [Polly](https://polly.llvm.org/), LLVM's polyhedral loop optimizer, is also included for users who want to experiment with additional optimization. Note that this toolchain is **not** suitable for anything other than bare-metal development; it has not been built with support for any libc or userspace development in mind.
+
+[binutils](https://www.gnu.org/software/binutils/) is also included for convenience. Unlike LLVM, however, the latest **stable** version of binutils is always used to reduce the opportunity for breakage because this project is primarily focused on cutting-edge Clang, not binutils. This means that **users do not need to download separate GCC toolchains** to build the Linux kernel.
+
+Automated builds occur weekly at 3 PM PST on every Saturday using fresh sources from the [LLVM Git monorepo](https://github.com/llvm/llvm-project). If any part of the builds fail, this repository will not be updated. The build scripts (powered by [tc-build](https://github.com/ClangBuiltLinux/tc-build)) behind this can be found [here](https://github.com/kdrag0n/proton-clang-build).
+
+Build notifications and other information can be obtained from the [Telegram channel](https://t.me/proton_clang_ci).
+
+## Host compatibility
+
+This toolchain is built on Ubuntu 18.04 LTS, which uses glibc 2.27. Compatibility with older distributions cannot be guaranteed. Other libc implementations (such as musl) are not supported.
+
+## Building Linux
+
+Make sure you have this toolchain in your `PATH`:
+
+```bash
+export PATH="$HOME/toolchains/proton-clang/bin:$PATH"
+```
+
+For an AArch64 cross-compilation setup, you must set the following variables. Some of them can be environment variables, but some must be passed directly to `make` as a command-line argument. It is recommended to pass **all** of them as `make` arguments to avoid confusing errors:
+
+- `CC=clang` (must be passed directly to `make`)
+- `CROSS_COMPILE=aarch64-linux-gnu-`
+- If your kernel has a 32-bit vDSO: `CROSS_COMPILE_ARM32=arm-linux-gnueabi-`
+
+Optionally, you can also choose to use as many LLVM tools as possible to reduce reliance on binutils. All of these must be passed directly to `make`:
+
+- `AR=llvm-ar`
+- `NM=llvm-nm`
+- `OBJCOPY=llvm-objcopy`
+- `OBJDUMP=llvm-objdump`
+- `STRIP=llvm-strip`
+
+Note, however, that additional kernel patches may be required for these LLVM tools to work. It is also possible to replace the binutils linkers (`lf.bfd` and `ld.gold`) with `lld` and use Clang's integrated assembler for inline assembly in C code, but that will require many more kernel patches and it is currently impossible to use the integrated assembler for *all* assembly code in the kernel.
+
+Android kernels older than 4.14 will require patches for compiling with any Clang toolchain to work; those patches are out of the scope of this project. See [android-kernel-clang](https://github.com/nathanchance/android-kernel-clang) for more information.
+
+### Differences from other toolchains
+
+Proton Clang has been designed to be easy-to-use compared to other toolchains, such as [AOSP Clang](https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/). The differences are as follows:
+
+- `CLANG_TRIPLE` does not need to be set because we don't use AOSP binutils
+- `LD_LIBRARY_PATH` does not need to be set because we set library load paths in the toolchain
+- No separate GCC/binutils toolchains are necessary; all tools are bundled