docs: add module documentation

src/lib.rs

@@ -1,3 +1,38 @@
+//! tokio-ping is an asynchronous ICMP pinging library.
+//!
+//! The repository is located at https://github.com/knsd/tokio-ping/.
+//!
+//! # Usage example
+//!
+//! Note, sending and receiving ICMP packets requires privileges.
+//!
+//! ```
+//! extern crate futures;
+//! extern crate tokio_core;
+//! extern crate tokio_ping;
+//!
+//! use futures::Stream;
+//!
+//! fn main() {
+//!     let addr = std::env::args().nth(1).unwrap().parse().unwrap();
+//!
+//!     let mut reactor = tokio_core::reactor::Core::new().unwrap();
+//!     let pinger = tokio_ping::Pinger::new(&reactor.handle()).unwrap();
+//!     let stream = pinger.chain(addr).stream();
+//!
+//!     let future = stream.take(3).for_each(|mb_time| {
+//!         match mb_time {
+//!             Some(time) => println!("time={}", time),
+//!             None => println!("timeout"),
+//!         }
+//!         Ok(())
+//!     });
+//!
+//!     reactor.run(future).unwrap_or_default();
+//! }
+//!
+//! ```
+
 #[macro_use] extern crate error_chain;
 extern crate futures;
 extern crate lazy_socket;