Add more guidance on package identifier.
Add more guidance on package identifier.

openspending/openspending#965

openspending/os-packager,openspending/fiscal-data-packager,kravets-levko/os-packager,openspending/fiscal-data-packager,kravets-levko/os-packager,openspending/os-packager,kravets-levko/os-packager