Specify the media type for proto representations (#346)

* Specify the media type for proto representations

Signed-off-by: Rachel Myers <rachelmyers@google.com>

* Add `MUST` to media types in proto and json

Signed-off-by: Rachel Myers <rachelmyers@google.com>

Author: rachelmyers

json-format.md

@@ -119,7 +119,7 @@ The CloudEvents [JSONSchema](http://json-schema.org) for the spec is located
 
 Each CloudEvents event can be wholly represented as a JSON object.
 
-Such a representation uses the media type `application/cloudevents+json`.
+Such a representation MUST use the media type `application/cloudevents+json`.
 
 All REQUIRED and all not omitted OPTIONAL attributes in the given event MUST
 become members of the JSON object, with the respective JSON object member name

protobuf-format.md

@@ -88,6 +88,7 @@ The CloudEvents type system MUST be mapped into the fields of
 | Integer      | int_value
 | Any          | Not applicable. Any is the enclosing CloudEventAny message itself
 
+Protocol Buffer representations of CloudEvents MUST use the media type `application/cloudevents+proto`.
 
 ## 3. Examples