Updating readme for more possible use cases

No documentation indicated that output when using `-h` changed based on the placement. For instance:

    $ dandelion -h deploy
    Usage: dandelion [options] <command> [<args>]
        -v, --version                    Display the current version
        -h, --help                       Display this screen
            --repo=[REPO]                Use the given repository
            --config=[CONFIG]            Use the given configuration file
  
      Available commands:
          deploy
          status

and

    $ dandelion deploy -h
    Usage dandelion deploy [options] [<revision>]
        --dry-run                       Show what would have been deployed


This also indicates `--dry-run` as an option when using `deploy`, which was highly valuable as I was getting ready to submit a pull request with this feature when I noticed it already existed.

SSH public key auth went undocumented so a note was added to the SFTP adapter to indicate how it was possible. This also provides more context for why the `password` param is sometimes optional.