Doc suggestion: add a glossary

[nicholasbishop]

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.

[casey]

This would be great, especially something which documented all the types and their relationships, such as Codec, Dispatch, the various Protos, etcetera.

[aturon]

Great idea!

[carllerche]

I moved this issue to the website repo: tokio-rs/website#62