This adds the first Tower guide called "Inventing the `Service` trait". It attempts to motivate all the parts to `Service` by walking the user through how they could have invented `Service` themselves, from scratch. It goes into quite a bit of detail but hopefully it paints a somewhat complete picture in the end.
The next guide I want to write is about how to implement a proper `Timeout` middleware using `Layer`, pin-project, and all the bells and whistles.
Ref: https://github.com/tower-rs/tower/issues/33
