Hyper: Clarify use of `#[tokio::main]`

[hwchen]

The #[tokio::main] macro is used in most (all?) of the examples, and also shows up in documentation.

However, it's not a part of the hyper crate (and was explicitly removed from reexport https://github.com/hyperium/hyper/issues/1878).

I feel like this could be a confusing spot for people new to hyper/tokio, perhaps it would be good to mention pulling in tokio as a dependency for this feature more explicitly.

[hwchen]

Now the cargo dep should be:

tokio = { version = "0.2.3", features = ["macros"] }

[kellytk]

Related issue: https://github.com/hyperium/hyper/issues/1613

[seanmonstar]

I've updated the examples README about dependencies required.