Document how to use coop threads (#654)

This is all off-by-default right now but the distribution supports this
through a separate sysroot. Document what's going on for others to
discover.

CoopThreading.md

@@ -0,0 +1,46 @@
+# Support for Cooperative Threading
+
+The component model specification is in the process of adding and defining
+intrinsics for guests to use to implement cooperative multithreading. The goal
+of this effort is to enable compiling applications using `pthread_*` style
+threading APIs, for example, to work with components. These intrinsics are
+described in the [component model explainer][explain] and this feature
+corresponds to the 🧵 emoji. The Wasmtime runtime supports this proposal behind
+the `-Wcomponent-model-threading` flag.
+
+[explain]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md
+
+Due to this feature being experimental and not officially included in any
+published WASI version yet it is off-by-default in wasi-sdk. By default
+`pthread_*`-style APIs either return an error (such as when spawning a thread)
+or trap (such as when blocking on a condition variable). Note that for
+compatibility, though, all `pthread_*` APIs are always available. The wasi-sdk
+distribution, however, does enable effectively swapping out these
+implementations in an opt-in basis to test out cooperative multithreading.
+
+Assuming that you've downloaded wasi-sdk and unpacked it within `$WASI_SDK_PATH`
+there is a sysroot located at `$WASI_SDK_PATH/share/wasi-sysroot/experimental-coop-threads`
+which can be used to experiment with cooperative multithreading. You can use
+this in your project like so:
+
+```console
+$WASI_SDK_PATH/bin/wasm32-wasip3-clang foo.c -o foo.wasm --sysroot \
+    $WASI_SDK_PATH/share/wasi-sysroot/experimental-coop-threads -pthread
+```
+
+Note the `--sysroot` and `-pthread` flags to indicate that threading is being
+used. When combined these flags should produce a binary which can then be run in
+Wasmtime, for example, like so:
+
+```console
+wasmtime -W component-model-threading ./foo.wasm
+```
+
+The alternative sysroot has support for `pthread_*` symbols in wasi-libc which
+additionally means support in C++ through the `<thread>` header, for example.
+
+It is intended in the near-ish future (before the end of 2026) that this
+experimental sysroot will become the default and wasi-libc/wasi-sdk will support
+cooperative multithreading by default. This decision will require the WASI
+subgroup to vote on including the cooperative multithreading feature in a future
+WASI release, and then wasi-libc/wasi-sdk will update to this release.