Add info about creating showback funding sources

If a `FundingSource` resource with the funding string exists in the Billing Service, its `id` property can be used in a `Charge` resource.

The Billing Service validates `FundingSource` resources against CBS.

If a `FundingSource` resource is not `active` in CBS, `Charge` resources using it will fail the submission process.

If a `FundingSource` resource is not `active`, `Charge` resources using it will fail the submission process.

When creating Showback/Chargeback charges, they need to use the appropriate Showback/Chargeback `FundingSource`.

Billing Service clients are able to create (`POST /fundingSource`) Showback `FundingSource` resources as they see fit for reporting purposes.

However, clients are not allowed to create Chargeback `FundingSource` resources since CBS is the source of truth for this.

__Example__

Request body for `FundingSource` resource.

```

{

  "udds": "A001001",

  "dnumber": "0"

}

```

#### Service

The `Service` resource represents the service a bill is charged for.

The Billing Service contains an initial set of services registered in CBS.