Documentation: add non-option parameter

[dnebauer]

This Git::Wrapper newbie spent time untangling two Git::Wrapper behaviours to understand why git->clone wasn't doing what he expected. Number 1: I wasn't sure how to add non-option parameters, i.e., a parameter on the command line that is not associated with an --option. An example is the repo url in git clone $repo_url. I blundered around trying every combination of hashs and lists before stumbling across #97 which made it clear in an example. The cpan/module documentation only covers options added with or without values. Perhaps you could help newbies out by explicitly explaining there how to add a non-option parameter. These teeting problems aside, thanks for a great module.