Docs: "why not green threads?"

[steveklabnik]

A lot of people wonder why tokio is made the way that it is. Especially with regards to other languages. It'd be worth having some sort of page that lays out the details here, so that someone can understand the basic design in comparison to others.

For example, there are some things that M:N threads do better than tokio, but those fall afoul of core Rust principles. Discussing this in an even-handed way would be very useful.

[steveklabnik]

I feel like @pcwalton could basically knock this out 😉

[Darksonn]

Closed in favor of #452.