Update README.md: added hyper-util to the dependencies section

[sheroz]

There is a need to include the hyper-util crate to compile examples, otherwise, it cannot solve the dependency for TokioIo. It looks like the examples also need to be updated as they refer to an internal "../benches/support/mod.rs" for TokioIo.

[seanmonstar]

I don't think the examples can actually depend on hyper-util, since the examples are part of of the hyper crate, and hyper-util depends on hyper. So it becomes circular. The only way we could is if the examples were made separate, with their own Cargo.toml file(s), then they could depend on both hyper and hyper-util.