Add helpful hint on io function for beginners

src/libstd/io/mod.rs

@@ -144,6 +144,16 @@
 //! # Ok(())
 //! # }
 //! ```
+//! Note that you cannot use the `?` operator in functions that do not return a `Result` (e.g. `main()`).
+//! Instead, you can `match` on the return value to catch any possible errors:
+//! 
+//! ```
+//! let mut input = String::new();
+//! match io::stdin().read_line(&mut input) {
+//!     Err(why) => panic!("Failed to read input: {}", why.description()),
+//!     Ok(_) => println!("You typed: {}", input.trim()),
+//! }
+//! ```
 //!
 //! And a very common source of output is standard output:
 //!