Add a basic README.md.

README.md

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+# WASI SDK
+
+## Quick Start
+
+[Download SDK packages here.](https://github.com/CraneStation/wasi-sdk/releases)
+
+## About this repository
+
+This repository contains no compiler or library code itself; it uses
+git submodules to pull in the upstream Clang and LLVM tree, as well as the
+WASI reference-sysroot tree.
+
+The Sysroot portion of this SDK is the
+[WASI reference-sysroot](https://github.com/CraneStation/reference-sysroot).
+
+Upstream Clang and LLVM 8.0 can compile for WASI out of the box, and WebAssembly
+support is included in them by default. So, all that's done here is to provide
+builds configured to set the default target and sysroot for convenience.
+
+One could also use a standard Clang 8.0, build a sysroot from the sources
+mentioned above, and compile with
+"--target=wasm32-unknown-wasi --sysroot=/path/to/sysroot".