Touch up the echo examples

examples/echo.rs

@@ -1,4 +1,18 @@
 //! An echo server that just writes back everything that's written to it.
+//!
+//! If you're on unix you can test this out by in one terminal executing:
+//!
+//! ```sh
+//! $ cargo run --example echo
+//! ```
+//!
+//! and in another terminal you can run:
+//!
+//! ```sh
+//! $ nc localhost 8080
+//! ```
+//!
+//! Each line you type in to the `nc` terminal should be echo'd back to you!
 
 extern crate env_logger;
 extern crate futures;