docs(service): add example of impl Service (#2209)

Add `examples/service_struct_impl.rs`, which provides an up-to-date
example of implementing MakeService and Service on custom types.
The `Svc` struct has a Counter, which demonstrates how to instantiate
shared resources in the `MakeSvc` and pass the resource to the
services. Updates the `examples/README.md` and the doc in
`src/service/mod.rs`.

Closes #1691

src/service/mod.rs

@@ -21,8 +21,9 @@
 //! to a single connection. It defines how to respond to **all** requests that
 //! connection will receive.
 //!
-//! While it's possible to implement `Service` for a type manually, the helper
-//! [`service_fn`](service_fn) should be sufficient for most cases.
+//! The helper [`service_fn`](service_fn) should be sufficient for most cases, but
+//! if you need to implement `Service` for a type manually, you can follow the example
+//! in `service_struct_impl.rs`.
 //!
 //! # MakeService
 //!