README, Makefile: update testing instructions, add make golden target

Makefile

@@ -1,8 +1,14 @@
 wit_files = $(sort $(shell find testdata -name '*.wit' ! -name '*.golden.*'))
 
+# json recompiles the JSON intermediate representation test files.
 .PHONY: json
 json: $(wit_files)
 
 .PHONY: $(wit_files)
 $(wit_files):
 	wasm-tools component wit -j --all-features $@ > $@.json
+
+# golden recompiles the .golden.wit test files.
+.PHONY: golden
+golden: json
+	go test ./wit -update

README.md

@@ -61,12 +61,31 @@ This will emit JSON on `stdout`, which can be piped to a file or another program
 wasm-tools component wit -j --all-features example.wit > example.wit.json
 ```
 
-## License
+## Contributing
 
-This project is licensed under the Apache 2.0 license with the LLVM exception. See [LICENSE](LICENSE) for more details.
+Developing and testing `wit-bindgen-go` requires an up-to-date installation of [Go](https://go.dev), [TinyGo](https://tinygo.org), [Wasmtime](https://wasmtime.dev), and [`wasm-tools`](https://crates.io/crates/wasm-tools).
 
-### Contributing
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be licensed as above, without any additional terms or conditions.
 
-Developing and testing `wit-bindgen-go` requires an up-to-date installation of [Go](https://go.dev), [TinyGo](https://tinygo.org), and [`wasm-tools`](https://crates.io/crates/wasm-tools).
+### Testing
 
-Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be licensed as above, without any additional terms or conditions.
+Tests are supported under both Go and TinyGo, on Linux, macOS, and WebAssembly.
+
+```sh
+go test ./...
+tinygo test ./...
+```
+
+Testing with WebAssembly (`wasip1`) requires an installation of [`go_wasip1_wasm32_exec`](https://go.dev/blog/wasi) and [Wasmtime](https://wasmtime.dev). WASI 0.2 `wasip2` is supported under TinyGo version 0.33.0 or later.
+
+```sh
+GOARCH=wasm GOOS=wasip1 go test ./...
+GOARCH=wasm GOOS=wasip1 tinygo test ./...
+tinygo test -target=wasip2 ./... # requires TinyGo 0.33.0 or later
+```
+
+This package contains tests that validate the round-trip parsing and re-serialization of WIT files back into WIT. To rebuild the test files in the [testdata](./testdata) directory, run `make golden`. This will reregenerate the JSON intermediate representation and the equivalent `*.golden.wit` files.
+
+## License
+
+This project is licensed under the Apache 2.0 license with the LLVM exception. See [LICENSE](LICENSE) for more details.