Add comment to README

[masaaki-horie]

Added comments to complement ambiguous task expressions.
Since this task expression conflicts with the standard optional argument expression in the CLI, a concrete example was added to the comment.

Check about standard cli usage syntax: http://docopt.org/

[ikari7789]

Is this necessary? The example already given works as well by just replacing :command_name with :list.

invoke 'laravel:artisan[:list]'

[masaaki-horie]

I made mistake like this, executing php artisan down --secret="secret-path" on remote servers.

cap env deploy laravel:artisan:down --secret="secret-path"

Correct command is this.

cap env deploy laravel:artisan[down --secret="secret-path"]

So I think need this extra comment. How do you think about this command structure?