Add CONTRIBUTING.md (WebAssembly#26)

It is not especially clear how to update the WIT ABI file, so this new
document describes the current steps to do so.

Author: abrown

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contributing
+
+Thank you for contributing to this specification. This repository follows the
+same guidelines as the [WASI contribution guidelines].
+
+[WASI contribution guidelines]: https://github.com/WebAssembly/WASI/blob/main/Contributing.md
+
+The specification is written in WIT (WebAssembly Interface Types) syntax in code
+blocks inside a Markdown file (`wasi-nn.wit.md`). This is parseable by WIT tools
+(e.g., [`wit-bindgen`], [`wit-abi`]). Note that, when altering the WIT
+specification, the ABI file (`wasi-nn.abi.md`) must also be updated or CI will
+fail. To use [`wit-abi`] to update the ABI file, run:
+
+[`wit-bindgen`]: https://github.com/bytecodealliance/wit-bindgen
+[`wit-abi`]: https://github.com/WebAssembly/wasi-tools/tree/main/wit-abi
+
+```console
+$ git clone https://github.com/WebAssembly/wasi-tools
+$ cd wasi-tools/wit-abi
+$ cargo build
+$ target/debug/wit-abi <path to wasi-nn directory>
+```