[skip ci] Add Plugins/PackageToJS/README.md

Author: kateinoigakukun

Diff for: CONTRIBUTING.md

@@ -59,10 +59,18 @@ Thank you for considering contributing to JavaScriptKit! We welcome contribution
 
 ### Running Tests
 
+Unit tests running on WebAssembly:
+
 ```bash
 make unittest SWIFT_SDK_ID=wasm32-unknown-wasi
 ```
 
+Tests for `PackageToJS` plugin:
+
+```bash
+swift test --package-path ./Plugins/PackageToJS
+```
+
 ### Editing `./Runtime` directory
 
 The `./Runtime` directory contains the JavaScript runtime that interacts with the JavaScript environment and Swift code.

Diff for: Plugins/PackageToJS/README.md

@@ -0,0 +1,43 @@
+# PackageToJS
+
+A Swift Package Manager plugin that facilitates building and packaging Swift WebAssembly applications for JavaScript environments.
+
+## Overview
+
+PackageToJS is a command plugin for Swift Package Manager that simplifies the process of compiling Swift code to WebAssembly and generating the necessary JavaScript bindings. It's an essential tool for SwiftWasm projects, especially those using JavaScriptKit to interact with JavaScript from Swift.
+
+## Features
+
+- Build Swift packages for WebAssembly targets
+- Generate JavaScript wrapper code for Swift WebAssembly modules
+- Support for testing Swift WebAssembly code
+- Diagnostic helpers for common build issues
+- Options for optimization and debug information management
+
+## Requirements
+
+- Swift 6.0 or later
+- A compatible WebAssembly SDK
+
+## Internal Architecture
+
+PackageToJS consists of several components:
+- `PackageToJSPlugin.swift`: Main entry point for the Swift Package Manager plugin (Note that this file is not included when running unit tests for the plugin)
+- `PackageToJS.swift`: Core functionality for building and packaging
+- `MiniMake.swift`: Build system utilities
+- `ParseWasm.swift`: WebAssembly binary parsing
+- `Preprocess.swift`: Preprocessor for `./Templates` files
+
+## Internal Testing
+
+To run the unit tests for the `PackageToJS` plugin, use the following command:
+
+```bash
+swift test --package-path ./Plugins/PackageToJS
+```
+
+Please define the following environment variables when you want to run E2E tests:
+
+- `SWIFT_SDK_ID`: Specifies the Swift SDK identifier to use
+- `SWIFT_PATH`: Specifies the `bin` path to the Swift toolchain to use
+