message: Update `README` file of `websocket` example
diff: A simple example that keeps a WebSocket connection open to an echo server.
  -The __[`main`]__ file contains all the code of the example.
  +The example consists of 3 modules:
  +- [`main`] contains the `Application` logic.
  +- [`echo`] implements a WebSocket client for the [`echo::server`] with `async-tungstenite`.
  +- [`echo::server`] implements a simple WebSocket echo server with `warp`.
  You can run it with `cargo run`:
  ```
  @@ -10,3 +13,5 @@ cargo run --package websocket
  ```
  [`main`]: src/main.rs
  +[`echo`]: src/echo.rs
  +[`echo::server`]: src/echo/server.rs
  