Downloads API docs

[acgeospatial]

Hi,

Could you expand on the readme.md https://github.com/OrdnanceSurvey/osdatahub/blob/master/README.md of the downloads section please?

'Note: For Premium Data Packages, this query will only return datasets if you have previously ordered the dataset on the OS Data Hub Website'

After this note I then see an example call to download opendata (great!), but can you also share the way to download a premium data package please - or point in direction where this is shown/documented so end user can have a simple example?

thanks!
Andrew