Add mandatory text specifying the content of the Discovery API response (fixes #214) (#360)

Author: bvdh

docs/specification/1.0.md

@@ -38,7 +38,7 @@ The discovery endpoint SHALL always be available at `{baseUrl}/cds-services`. Fo
 
 ### Response
 
-The response to the discovery endpoint is an object containing a list of CDS Services.
+The response to the discovery endpoint SHALL be an object containing a list of CDS Services.
 
 Field | Description
 ----- | ---------