Please document license of libnftnl, not just the license of the bindings

[joshtriplett]

Even if the bindings use MIT/Apache-2.0 like the rest of the Rust ecosystem, the underlying libnftnl library is GPLv2-or-later, and any Rust program using this crate would be subject to that license (in addition to the license of the bindings). Could you please add that documentation to the README, and the corresponding license information to Cargo.toml?