Add note about using Decodable instead of a custom trait.
Add note about using Decodable instead of a custom trait.

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