winch: Add high-level project docs

This commit introduces a REAME.md in `winch`s directory with some
high-level documentation which was long overdue.

winch/README.md

@@ -0,0 +1,69 @@
+<div align="center">
+  <h1>Winch</h1>
+
+  <h3>WebAssembly Intentionally Non-optimizing Compiler and Host</h3>
+
+  <p>
+    <strong>A WebAssembly baseline compiler</strong>
+  </p>
+
+  <strong>A <a href="https://bytecodealliance.org/">Bytecode Alliance</a> project</strong>
+
+  <p>
+    <a href="https://github.com/bytecodealliance/wasmtime/actions?query=workflow%3ACI"><img src="https://github.com/bytecodealliance/wasmtime/workflows/CI/badge.svg" alt="build status" /></a>
+    <a href="https://bytecodealliance.zulipchat.com/#narrow/stream/417703-winch"><img src="https://img.shields.io/badge/zulip-join_chat-brightgreen.svg" alt="zulip chat" /></a>
+    <img src="https://img.shields.io/badge/rustc-stable+-green.svg" alt="supported rustc stable" />
+    <a href="https://docs.rs/winch-codegen"><img src="https://docs.rs/winch-codegen/badge.svg" alt="Documentation Status" /></a>
+  </p>
+</div>
+
+## About
+
+Winch is a WebAssembly "baseline" or single-pass compiler designed for Wasmtime.
+
+Winch's primary goal is compilation performance, therefore only certain, very
+limited peephole optimations are applied.
+
+For more details on the original motivation and goals, refer to the [Bytecode
+Alliance RFC for Baseline Compilation in Wasmtime.](rfc).
+
+[rfc]: https://github.com/bytecodealliance/rfcs/blob/main/accepted/wasmtime-baseline-compilation.md
+
+## Design principles
+
+* Single pass over Wasm bytecode
+
+* Function as the unit of compilation
+
+* Machine code generation directly from Wasm bytecode – no intermediate
+  representation
+
+* Avoid reinventing machine-code emission – use Cranelift's instruction emitter
+  code to create an assembler library
+
+* Prioritize compilation performance over runtime performance
+
+* Simple to verify by looking. It should be evident which machine instructions
+  are emitted per WebAssembly operator
+
+* Adding and iterating on new (WebAssembly and developer-facing) features should
+  be simpler than doing it in an optimizing tier (Cranelift)
+
+
+## Status
+
+Winch's aim is to support all the backends officially supported by Wasmtime:
+
+* x64\_64
+* arm64
+* riscv64
+* s390x
+
+The x86\_64 backend offers an almost-complete implementation, it currently
+supports all the instructions that are part of WebAssembly's MVP, plus some of
+the [feature extensions](feature-extensions). Refer to the [Tiers of
+Support](tiers-of-support) for more details.
+
+
+[feature-extensions]: https://webassembly.org/features/
+[tiers-of-support]: https://docs.wasmtime.dev/stability-tiers.html