Add a toolchain for toolchain building

kotlin-native/tools/toolchain_builder/README.md

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+# Kotlin/Native toolchains builder
+
+This directory contains a set of scripts and configuration files that allow one to build the same toolchain that is used by Kotlin/Native.
+
+### System requirements
+* Docker
+
+### Usage
+1. First, you need to build a Docker image with crosstool-ng inside. Use `create_image.sh` for it.
+2. Now you can build an actual toolchain. To pick one, take a look inside `toolchains` folder. 
+It is organized as `$TARGET/$VERSION`. Then run `./run_container.sh $TARGET $VERSION`. Building a toolchain might take a while (~17 minutes on Ryzen 5 3600). 
+Once ready, an archive will be placed in `artifacts` folder.
+
+### Example
+```bash
+./create_image.sh && ./run_container.sh aarch64-unknown-linux-gnu gcc-8.3.0-glibc-2.25-kernel-4.9
+```
+
+### Notes
+Check that Docker can write to `artifacts` directory (where `build_toolchains.sh` places archives).