Add a note on WebAssembly to README.md

Author: Xaeroxe

README.md

@@ -9,6 +9,22 @@ A Rust equivalent of Unix command "which". Locate installed executable in cross
 * Linux
 * Windows
 * macOS
+* wasm32-wasi*
+
+### A note on WebAssembly
+
+This project aims to support WebAssembly with the [wasi](https://wasi.dev/) extension. This extension is a requirement. `which` is a library for exploring a filesystem, and
+WebAssembly without wasi does not have a filesystem. `which` cannot do anything useful without this extension. Issues and PRs relating to
+`wasm32-unknown-unknown` and `wasm64-unknown-unknown` will not be resolved or merged. All `wasm32-wasi*` targets are officially supported.
+
+If you need to add a conditional dependency on `which` for this reason please refer to [the relevant cargo documentation for platform specific dependencies.](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies)
+
+Here's an example of how to conditionally add `which`. You should tweak this to your needs.
+
+```toml
+[target.'cfg(not(all(target_family = "wasm", target_os = "unknown")))'.dependencies]
+which = "6.0.3"
+```
 
 ## Examples