High level guide for Tide

Currently, if a new user wanted to understand some of the core ideas behind Tide, they would have to find their way to the following blogposts:

* https://rust-lang-nursery.github.io/wg-net/2018/10/16/tide-routing.html
* https://rust-lang-nursery.github.io/wg-net/2018/09/11/tide.html
* https://rust-lang-nursery.github.io/wg-net/2018/11/07/tide-middleware.html

I think that we need to collect the information presented in these posts and flesh them out as a proper guide talking about the concepts of Tide. This in conjunction with more examples can serve as the initial version of documentation for the framework.