Add mandatory wording to state Discovery endpoint SHALL use /cds-services (fixes #213) (#359)

Author: bvdh

docs/specification/1.0.md

@@ -32,7 +32,7 @@ A CDS Service provider SHALL expose its Discovery endpoint at"
 ```
 ### HTTP Request
 
-The discovery endpoint is always available at `{baseUrl}/cds-services`. For example, if the `baseUrl` is https://example.com, the EHR would invoke:
+The discovery endpoint SHALL always be available at `{baseUrl}/cds-services`. For example, if the `baseUrl` is https://example.com, the EHR would invoke:
 
 `GET https://example.com/cds-services`