Document supported set of target triples

[james-rms]

The README includes examples that tell the user to specify --target=wasm32-wasi, but that's not the only option available. There are cursory references to wasm32-wasip1, wasm32-wasip2, and wasm32-wasip1-threads in the Limitations section of the README. It's not clear to me what other target triples are meant to be supported in a given WASI SDK release. The answer must depend on the sysroot and upstream clang.

Also note that the wasm32-wasi target was deprecated in rustc, and I think the intention here is to keep these similar or the same? I'm not sure.

[alexcrichton]

Supported targets right now are:

• wasm32-wasi - the exact same as wasm32-wasip1. Intention is to deprecate/remove, but work has not gone into this yet.
• wasm32-wasip1
• wasm32-wasip2
• wasm32-wasi-threads - intention is to deprecate/remove like above to recommend wasm32-wasip1-threads instead.
• wasm32-wasip1-threads

Agreed this should be documented!