feat(ffi): Initial C API for hyper

capi/README.md

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+# C API for hyper
+
+This provides auxiliary pieces for a C API to use the hyper library.
+
+## Unstable
+
+The C API of hyper is currently **unstable**, which means it's not part of the semver contract as the rest of the Rust API is.
+
+Because of that, it's only accessible if `--cfg hyper_unstable_ffi` is passed to `rustc` when compiling. The easiest way to do that is setting the `RUSTFLAGS` environment variable.
+
+## 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:
+
+```
+RUSTFLAGS="--cfg hyper_unstable_ffi" cargo build --features client,http1,http2,ffi
+```