Only Works on *Public* Datasets/Packages

There should be a mention in the README, or somewhere else like the documentation for package_show, that this only works for datasets (aka packages) that have been made public. package_show will only return public datasets, therefore if you attempt to use this script to upload files to a private dataset, it fails with a message that it can't find the dataset.

It would be very helpful if there were at least a mention of that somewhere. If that is tied to a config setting somewhere that we've set for datasets on our CKAN instance, that also could be very good to make users aware of.