review: add top-level README with embedder warning

Author: abrown

crates/wasi-threads/README.md

@@ -0,0 +1,12 @@
+# wasmtime-wasi-threads
+
+Implement the `wasi-threads` [specification] in Wasmtime.
+
+[specification]: https://github.com/WebAssembly/wasi-threads
+
+> Note: this crate is experimental and not yet suitable for use in multi-tenant
+> embeddings. As specified, a trap or WASI exit in one thread must end execution
+> for all threads. Due to the complexity of stopping threads, however, this
+> implementation currently exits the process entirely. This will work for some
+> use cases (e.g., CLI usage) but not for embedders. This warning can be removed
+> once a suitable mechanism is implemented that avoids exiting the process.