Remove Job from implemented entities list

It's not necessary to list Job as unimplemented, as Customers are now Jobs if certain attributes are set (specifically, `Job` and `ParentRef`). These attributes are already implemented in the Customer model, so this README change is all that's really needed.

For more info, see the [QBO V2 to V3 Migration Guide](https://developer.intuit.com/docs/0025_quickbooksapi/0058_faq/qbo_v2_to_v3_migration_guide#Jobs_%28subcustomers%29).