chore(adapters): add README (#9059)

This commit adds a README to the
`wasi-preview1-component-adapter-provider` crate to make it clearer
how to use it and what it's for.

While the README is mostly for those who are already familiar, a
section giving a brief overview of context around the adapters was
added.

Signed-off-by: Victor Adossi <vadossi@cosmonic.com>

crates/wasi-preview1-component-adapter/provider/README.md

@@ -0,0 +1,77 @@
+<div align="center">
+  <h1><code>wasi-preview1-component-adapter-provider</code></h1>
+  <strong>A <a href="https://bytecodealliance.org/">Bytecode Alliance</a> project</strong>
+  <p>
+    <strong>
+      A utility library containing binaries for WASI Preview1 adapters for easy use from Rust.
+    </strong>
+  </p>
+
+  <p>
+    <a href="https://crates.io/crates/wasi-preview1-component-adapter-provider"><img src="https://img.shields.io/crates/v/wasi-preview1-component-adapter-provider.svg?style=flat-square" alt="Crates.io version" /></a>
+    <a href="https://crates.io/crates/wasi-preview1-component-adapter-provider"><img src="https://img.shields.io/crates/d/wasi-preview1-component-adapter-provider.svg?style=flat-square" alt="Download" /></a>
+    <a href="https://docs.rs/wasi-preview1-component-adapter-provider/"><img src="https://img.shields.io/badge/docs-latest-blue.svg?style=flat-square" alt="docs.rs docs" /></a>
+  </p>
+</div>
+
+`wasi-preview1-component-adapter-provider` contains the raw bytes of the WASI Preview1 to Preview2 adapters (Reactor, Command, and Proxy).
+
+For example, if you wanted to write the adapter bytes back into a `.wasm` binary:
+
+```rust
+use wasi_preview1_component_adapter_provider::WASI_SNAPSHOT_PREVIEW1_REACTOR_ADAPTER;
+
+fn main() {
+    std::fs::write(
+        "wasi_snapshot_preview1.reactor.wasm",
+        WASI_SNAPSHOT_PREVIEW1_REACTOR_ADAPTER
+    ).expect("failed to write bytes to file");
+}
+```
+
+A more realistic use-case is performing the *adaptation* step of preparing a WASI Preview2 component from an existing WASI Preview1 component:
+
+```rust
+use wasi_preview1_component_adapter_provider::WASI_SNAPSHOT_PREVIEW1_REACTOR_ADAPTER;
+use wit_component::ComponentEncoder;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let wasm_p1_bytes = std::fs::read("path/to/your/your-component.p1.wasm");
+
+    let wasm_p2_bytes = ComponentEncoder::default()
+        .module(&wasm_module_bytes)?
+        .adapter(
+            "wasi_snapshot_preview1",
+            WASI_SNAPSHOT_PREVIEW1_REACTOR_ADAPTER,
+        )?
+        .validate(true)
+        .encode()?;
+
+    std::fs::write("your-component.p2.wasm", wasm_p2_bytes)?;
+
+    Ok(())
+}
+```
+
+## What is a component adapter?
+
+Code compiled to WebAssembly as described by the [base WebAssembly Spec][wasm-spec] is considered a WebAssembly "module" or "core module".
+
+To robustly support rich types, composition, and easier interoperability, the [Component Model][cm] was created and is the spec that governs the idea of a "WebAssembly component". The component model wraps any existing WebAssembly core module(s) and extends them with additional features.
+
+To standardize underlying system interoperability (ex. reading files, system time) in code compiled to WebAssembly, the [WebAssembly System Interface ("WASI")][wasi] was created. WASI is implemented by language tool chains (ex. Rust supports `wasm32-wasi`/`wasm32-wasip1` as a target, with [support for `wasm32-wasip2` on the way][rust-target-wasm32-wasi]), and enables compiling a WebAssembly component that utilizes [the interfaces that make up WASI Preview1][wasi-p1-interfaces].
+
+In the ongoing work of building WASI, WASI Preview2 which contains more functionality [has been released][wasi-p2-release] -- but building directly to Preview2 is not yet integrated into language toolchains. However, Preview1 components (which *can* be produced by curren toolchains) can be *adapted* to WASI Preview2.
+
+This is where component adapters come in.
+
+**Component Adapters are WebAssembly binaries that contain logic that can take any WebAssembly binary implemented in terms of WASI Preview1 and convert them to WASI Preview2.**
+
+This crate contains the binary content of those the adapter WebAssembly binaries, made accessible as constant arrays of bytes (`const &[u8]`).
+
+[wasm-spec]: https://webassembly.github.io/spec/core
+[cm]: https://component-model.bytecodealliance.org
+[wasi]: https://wasi.dev/
+[wasi-p2-release]: https://bytecodealliance.org/articles/WASI-0.2
+[rust-target-wasm32-wasi]: https://blog.rust-lang.org/2024/04/09/updates-to-rusts-wasi-targets.html
+[wasi-p1-interfaces]: https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/witx/wasi_snapshot_preview1.witx