Merge pull request #441 from cds-hooks/issue/254-cds-service-http-status-codes

Document possible HTTP status codes a CDS Service may return (fixes #254)

Author: brynrhodes

docs/specification/1.0.md

@@ -422,6 +422,15 @@ For successful responses, CDS Services SHALL respond with a 200 HTTP response wi
 
 > Note that because the CDS client may be invoking multiple services from the same hook, there may be multiple responses related to the same information. This specification does not address these scenarios specifically; both CDS Services and CDS Clients should consider the implications of multiple CDS Services in their integrations.
 
+### HTTP Status Codes
+
+Code | Description
+---- | -----------
+`200 OK` | A successful response.
+`412 Precondition Failed` | The CDS Service is unable to retrieve the necessary FHIR data to execute its decision support, either through a prefetch request or directly calling the FHIR server.
+
+CDS Services MAY return other HTTP statuses, specifically 4xx and 5xx HTTP error codes.
+
 ### Card Array
 
 Field | Optionality | Type | Description