Tokio vs Futures intro point

[AndyGauge]

https://github.com/tokio-rs/doc-push/issues/68#issuecomment-428614359 and prior discussion:

Sorry to butt in, but I think it would be nice to make a clear distinction about what tokio is versus what futures are. I don't know if it's really the job of tokio to do that kind of education but, as a beginner, learning tokio is the first time I've encountered futures in Rust so clearly defining tokio's relationship to/distinction from futures would be really helpful to me. - @rawmonstar

[rylev]

I think this is important. Putting it in the futures section makes sense since we're talking about futures, but putting something in the runtime section also makes sense since that's a large part of what Tokio is. I'll merge this for now, and we can see how it works out in practice.