Add example of `docopt!` macro to crate docs.
Add example of `docopt!` macro to crate docs.

alexcrichton/docopt.rs,alexcrichton/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,birkenfeld/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,brandonson/docopt.rs,brandonson/docopt.rs,zsiciarz/docopt.rs,brandonson/docopt.rs