Add minimum contribution guide

Author: kateinoigakukun

CONTRIBUTING.md

@@ -1,3 +1,28 @@
+# How to Contribute
+
+Thank you for your interest in contributing to ruby.wasm!
+This document describes development setup and pointers for diving into this project.
+
+## Install dependencies
+
+```console
+$ git clone https://github.com/ruby/ruby.wasm
+$ cd ruby.wasm
+$ bundle install
+$ rake --tasks
+```
+
+## Building and Testing [`ruby-wasm-wasi`](./packages/ruby-wasm-wasi)
+
+```console
+# Download a prebuilt Ruby release (if you don't need to re-build Ruby)
+$ rake build:download_prebuilt
+# Build npm package
+$ rake npm:ruby-head-wasm-wasi
+# Test npm package
+$ rake npm:ruby-head-wasm-wasi-check
+```
+
 ## Re-bindgen from `.wit` files
 
 If you update [`*.wit`](https://github.com/WebAssembly/component-model/blob/ed90add27ae845b2e2b9d7db38a966d9f78aa4c0/design/mvp/WIT.md), which describe the interface of a WebAssembly module, either imported or exported, you need to re-generate glue code from `*.wit`.

README.md

@@ -129,3 +129,8 @@ $ ls packages/npm-packages/ruby-head-wasm-wasi/ruby-head-wasm-wasi-*.tgz
 The current WASI target build does not yet support `Thread` related APIs. Specifically, WASI does not yet have an API for creating and managing threads yet.
 
 Also there is no support for networking. It is one of the goal of WASI to support networking in the future, but it is not yet implemented.
+
+
+## Contributing
+
+Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/ruby.wasm