message: docs(ffi): document nightly requirement for FFI
  Update the command to use `--crate-type`.
diff: @@ -10,8 +10,8 @@ Because of that, it's only accessible if `--cfg hyper_unstable_ffi` is passed to
  ## Building
  -The C API is part of the Rust library, but isn't compiled by default. Using `cargo`, it can be compiled with the following command:
  +The C API is part of the Rust library, but isn't compiled by default. Using a nightly release of `cargo`, starting with `nightly-2022-03-02`, it can be compiled with the following command:
  ```
  -RUSTFLAGS="--cfg hyper_unstable_ffi" cargo build --features client,http1,http2,ffi
  +RUSTFLAGS="--cfg hyper_unstable_ffi" cargo +nightly rustc --features client,http1,http2,ffi -Z unstable-options --crate-type cdylib
  ```
  