Expand create-project --repository-url flag documentation.

I've had some trouble figuring out the correct format for a local repository to use with `create-project` (see http://stackoverflow.com/questions/16299713/composer-create-project-fails-on-local-package-repository).

I've had a look at the code (https://github.com/composer/composer/blob/master/src/Composer/Command/CreateProjectCommand.php#L144-L152) and documented the behavior.

Hope this helps.