Improve uploadDir `getS3Params` callback documentation

[ChrisBAshton]

The example code in the README is a little unclear at the moment:

function getS3Params(localFile, stat, callback) {
  // call callback like this:
  var err = new Error(...); // only if there is an error
  var s3Params = { // if there is no error
    ContentType: getMimeType(localFile), // just an example
  };
  // pass `null` for `s3Params` if you want to skip uploading this file.
  callback(err, s3Params);
}

Questions:

• what should the first parameter for callback be if there AREN'T any errors? (Right now we're passing hard-coded undefined as first parameter)
• what sort of errors are we expecting? Should I be checking the stat object for something? I'm assuming not...

Please consider changing to something like:

function getS3Params(localFile, stat, callback) {
  if (thereIsAProblem()) {
    callback(new Error("Some error message..."));
  }
  else if(weDoNotWantToUpload(localFile))) {
    callback(undefined, null);
  }
  else {
    var s3Params = {
      ContentType: getMimeType(localFile), // just an example
    };
    callback(undefined, s3Params);
  }
}