Include doc on wstd in the readme

Now that wstd is a hosted Bytecode Alliance project, I am looking for places to raise awareness about the project. I think we should link to it in our relevant rust projects like this one.

Perhaps even referencing how to adopt wstd with this commit: https://github.com/bytecodealliance/sample-wasi-http-rust/commit/017564bb2b468a72e38f8452244a6b4b9f8a567f