Add documentation on specifying a region / allowed region values

[Elyseum]

When creating a marketplace, non-US users will want to specify the region to create the marketplace in.
You can do this by specifying the --region / -r parameter, which is part of the --help output of the seed command.

What is not specified are the allowed values this parameter accepts. As far as I know, there is not document describing it. I managed to derive them from the filter list on https://portal.ordercloud.io/marketplaces.

Allowed values are:

• aus (for 'Australia-East')
• est (for 'US-East')
• eur (for 'Europe-West')
• jpn (for 'Japan-East')
• usw (for 'US-West', this is the default value)