Fix: #119. Mention interoperability considerations in registration.

Author: ioggstream

draft-ietf-httpapi-rest-api-mediatypes.md

@@ -320,6 +320,9 @@ Q: Why this document?
    This has some security implications too
    (eg. wrt on identifying parsers or treat downloads)
 
+Q: Do we support OpenAPI 2.0 / Swagger?
+:  No, this document is about OpenAPI 3.0 and above.
+
 # Change Log
 {: numbered="false" removeinrfc="true"}