Add documentation for importing from cargo.toml files

[thomasvanlankveld]

I am able to import code from cargo.toml, as described here: parcel-bundler/parcel#3186 (comment).

This behavior is not described in the Rust documentation: https://parceljs.org/rust.html.

[kena0ki]

I think that feature doesn't exist in Parcel, unless using plugins, such as: https://github.com/wasm-tool/parcel-plugin-wasm.rs.