Update readme with note about new onError() callback function
Update readme with note about new onError() callback function

FineUploader/fine-uploader,SimonWaldherr/uploader,SimonWaldherr/uploader,SimonWaldherr/uploader,SimonWaldherr/uploader,SimonWaldherr/uploader,FineUploader/fine-uploader,SimonWaldherr/uploader,FineUploader/fine-uploader,SimonWaldherr/uploader