Add a glossary

[carllerche]

I think that https://tokio.rs/docs/getting-started/tokio/ would benefit from a glossary of terms covering tokio-core, tokio-proto, tokio-service, and futures. I think a lot of the terms are already explained well in isolation on various pages, but a single page that brings all the definitions together would be helpful.

Copied from: tokio-rs/tokio-proto#127

[Darksonn]

We have a glossary now: https://tokio.rs/tokio/glossary