Clarify interoperability of extensions (#508)
Clarify interoperability of extensions (#508)

Clarified definition of extension attributes to separate:

1. Core event data model definition of extension attributes.
2. Bindings must serialize extension attributes.
3. Advice about defining extension attributes.

Signed-off-by: Alan Conway <f52ac7f18aee66f8da492e36fb0a920cfff785a4@redhat.com>

cloudevents/spec