Document how to use a Service factory for Server::serve()

[daxpedda]

I would like to wrap a user supplied app, like a Router, for that purpose I would like to use ServiceBuilder.

The issue is that Server::serve() needs a service factory, like what can be produced with Router::into_make_service(), which isn't available for ServiceBuilder.

The solution I found is wrapping it in Shared. I believe some documentation in axum-server would help.

See tokio-rs/axum#1203 for the original problem.

[programatik29]

I will add it for next release.

[daxpedda]

Added in 1a68546.