Guide: Standard input

FWIW, I don't personally regard `expect` and `unwrap` as something we should be encouraging for legitimate errors: in my mind they are essentially assertions that the `None` case never happens, since `fail!` isn't really recoverable at all.

I would prefer
```rust
let input = match io::stdin().read_line() {
    Ok(i) => i,
    Err(e) => {
        println!("Failed to read line: {}", e);
    }
};
```

Of course, this is more verbose, may introduce too much at a time...