Clarify versioning of CDEvents spec and events

[afrittoli]

Follow up to the discussion in #30 (comment)

We should clarify whether to we want to version events individually or not and what does that mean in terms of compliance to the spec for producers and consumers.

[m-linner-ericsson]

We might want to include something like following text as requirement for the version.

MUST be the version of the event/spec used for producing the event

[afrittoli]

Closing in favour of #43