Improve Web Image README

Author: patrick96

web-image/README.md

@@ -1,33 +1,76 @@
 # Web Image
 
+Web Image is an experimental backend for Native Image that produces a
+WebAssembly module from Java code.
+
+## Prerequisites
+
+Being part of Native Image, check out its [Quick Start Guide](../docs/reference-manual/native-image/contribute/DevelopingNativeImage.md)
+for all the prerequisites for building Native Image.
+
+In addition, Web Image also uses `wasm-as` from
+[binaryen](https://github.com/WebAssembly/binaryen) as its assembler.
+Version 119 of `wasm-as` has to be available on your `PATH`.
+
+<details>
+<summary>Installation</summary>
+
+Binaryen provides pre-built releases for all major platforms on
+[GitHub](https://github.com/WebAssembly/binaryen/releases).
+
+On MacOS, it is recommended to install through [Homebrew](https://brew.sh/), as
+the pre-built binaries are not signed and may be quarantined by MacOS.
+
+```bash
+brew install binaryen
+```
+</details>
+
 ## Building
 
+To build Web Image, run the `mx build` command in the `web-image` suite:
+
 ```bash
+cd /path/to/graal
+cd web-image
 mx build
 ```
 
 ## Usage
 
-Use this as you would regular the regular `native-image` tool, but with an
+Use this as you would use the regular `native-image` tool, but with an
 additional `--tool:svm-wasm` flag to enable the WebAssembly backend:
 
 ```bash
 mx native-image --tool:svm-wasm -cp ... HelloWorld
 ```
 
 This produces `helloworld.js` and `helloworld.js.wasm` in your working
-directory. The JavaScript file is a wrapper that loads and runs the WebAssembly
-code and can be run with [Node.js](https://nodejs.org/en) 22 or later:
+directory. The `--tool:svm-wasm` flag should be the first argument if possible.
+If any experimental options specific to the Wasm backend are used, they can
+only be added after the `--tool:svm-wasm` flag.
 
-The `--tool:svm-wasm` flag should be the first argument if possible. If any
-experimental options specific to the Wasm backend are used, they can only be
-added after the `--tool:svm-wasm` flag.
+The JavaScript file is a wrapper that loads and runs the WebAssembly
+code and can be run with [Node.js](https://nodejs.org/en) 22 or later:
 
 ```bash
-$ node helloworld.js
+# --experimental-wasm-exnref is only required for Node versions before 25
+$ node --experimental-wasm-exnref helloworld.js
 Hello World
 ```
 
+## WebAssembly Features
+
+The WebAssembly code generated by Web Image makes use of various WebAssembly
+features from WebAssembly 3.0:
+
+- [Garbage Collection](https://github.com/WebAssembly/gc)
+- [Exception Handling](https://github.com/WebAssembly/exception-handling/blob/master/proposals/exception-handling/Exceptions.md)
+- [Typed Function References](https://github.com/WebAssembly/function-references/blob/main/proposals/function-references/Overview.md)
+
+Support for [WebAssembly 2.0](https://www.w3.org/TR/wasm-core-2/#release-20) is
+generally assumed.
+
 ## Contributors
 
 - Aleksandar Prokopec