wasi-common: unclear license of witx files in WASI submodule

I'm working on packaging wasmtime for Fedora Linux, and during package review it came up that the wasi-common crate contains a submodule ("WASI") where it's entirely unclear how those files are licensed, especially the .witx files included from `WASI/phases/`.

Side note: It looks like lots of the `README.md` (and some `docs.md`) files from the `WASI` submodule are included by mistake, because `Cargo.toml` has `include = [..., "README.md", ...]` instead of `include = [..., "/README.md", ...]` (note the leading `/` to make sure only the `README.md` file from the crate root matches, not ***all*** `README.md` files in all child directories).

---

See also: https://github.com/WebAssembly/WASI/issues/182