Document the library about tower

[davidpdrsn]

Improving the general documentation around Tower is something I would like to drive. I have seen several cases on Twitter and Reddit where people say that Tower seems cool but they don't really understand it. I also remember thinking the same thing when I first came to Tower.

I'm very open to suggestions about how to do this but my current plan is:

1. Make sure all middlewares have examples in the docs. Trying to figure out how to use something from the types alone can be hard.
2. Add examples of how to put all the pieces together and build an HTTP server and client with hyper. Our current number of examples isn't impressive 😅
3. Write a guide similar to Tonic's helloworld guide that explains how to build a middleware from scratch.
4. Link to the Tower deep dive stream I did some months ago. Several people have mentioned this helped them understand the concepts better.

I don't imagine we can get all this done before Tower 0.5 so don't think it should block that release.

[davidbarsky]

Regarding “Implement Service for Arc<MyType> to get around &mut requirement“, could this be done using arbitrary self types, or am I misunderstanding something?