Add some guidance on how to construct CloudEvents (#404)

* Add some guidance on how to construct CloudEvents

Since not all event producers will generate CloudEvents natively, it might
be good to provide some guidance around how the CE attributes should be
populated when the CE producer isn't the event producer.

Signed-off-by: Doug Davis <dug@us.ibm.com>

* More edits

Signed-off-by: Doug Davis <dug@us.ibm.com>

* clarify split between source and producer better

Signed-off-by: Doug Davis <dug@us.ibm.com>

primer.md

@@ -274,6 +274,57 @@ serialization for unkown, or even new, properties. It was also noted that the
 HTTP specification is now following a similar pattern by no longer suggesting
 that extension HTTP headers be prefixed with `X-`.
 
+## Creating CloudEvents
+
+The CloudEvents specification purposely avoids being too prescriptive about
+how CloudEvents are created. For example, it does not assume that the original
+event source is the same entity that is constructing the associated
+CloudEvent for that occurrence. This allows for a wide variety of implementation
+choices. However, it can be useful for implementors of the specification
+to understand the expectations that the specification authors had in mind
+as this might help ensure interoperability and consistency.
+
+As mentioned above, whether the entity that generated the initial event is
+the same entity that creates the corresponding CloudEvent is an implementation
+choice. However, when the entity that is constructing/populating the
+CloudEvents attributes is acting on behalf of the event source, the values
+of those attributes are meant to describe the event or the event source
+and not the entity calculating the CloudEvent attribute values. In other words,
+when the split between the event source and the CloudEvents producer are
+not materially significant to the event consumers, the spec defined
+attributes would typically not include any values to indicate this split
+of responsibilities.
+
+This isn't to suggest that the CloudEvents producer
+couldn't add some additional attributes to the CloudEvent, but those
+are outside the scope of the interoperability defined attributes of the spec.
+This is similar to how an HTTP proxy would typically minimize changes to the
+well-defined HTTP headers of an incoming message, but it might add some
+additional headers that include proxy-specific metadata.
+
+It is also worth noting that this separation between original event source
+and CloudEvents producer could be small or large. Meaning, even if the
+CloudEvent producer were not part of the original event source's ecosystem,
+if it is acting on behalf of the event source, and its presence in the
+flow of the event is not meaningful to event consumers, then the above
+guidance would still apply.
+
+When an entity is acting as both a receiver and sender of CloudEvents
+for the purposes of forwarding, or transforming, the incoming event, the
+degree to which the outbound CloudEvent matches the inbound CloudEvent
+will vary based on the processing semantics of this entity. In cases where
+it is acting as proxy, where it is simply forwarding CloudEvents
+to another event consumer, then the outbound CloudEvent will typically
+look identical to the inbound CloudEvent with respect to the spec defined
+attributes - see previous paragraph concerning adding additional attributes.
+
+However, if this entity is performing some type of semantic processing
+of the CloudEvent, typically resulting in a change to the value of the
+`data` attribute, then it may need to be considered a distinct "event
+source" from the original event source. And as such, it is expected
+that CloudEvents attributes related to the event producer (such as 'source`
+and `id`) would be changed from the incoming CloudEvent.
+
 ## Qualifying Protocols and Encodings
 
 The explicit goal of the CloudEvents effort, as expressed in the specification,
@@ -661,7 +712,7 @@ existing current event formats that are used in practice today was gathered.
 
 #### AWS - CloudWatch Events
 
-A high proportion of event-processing systems on AWS are converging on 
+A high proportion of event-processing systems on AWS are converging on
 the use of this format.
 
 ```