Document engine flags for enabling wasi-nn

[abrown]

The documentation does not make it clear that currently one must build Wasmtime with a special flag --features wasi-nn in order to enable the wasi-nn features. This is present in the GitHub workflow but it should be made explicit somewhere (perhaps in the top-level README and linked to from the as and rust READMEs?).

[abrown]

The --features wasi-nn flag will no longer be necessary once the next version of Wasmtime is released (v0.27.0). After bytecodealliance/wasmtime#2859, wasi-nn can be enabled by passing the --wasi-modules=experimental-wasi-nn flag at runtime. After the next Wasmtime release, we should:

• document that the --wasi-modules flag is necessary,
• update this project's CI to use the newer version of Wasmtime
• and update the build.sh script to use the --wasi-modules flag.

[brianjjones]

Docs have been updated. And the CI uses the build.sh script to test, and that script has been updated with the needed flags.