Document how to work with experimental event types

[magnusbaeck]

Description

At the Oct 2022 summit we discussed deployment events and the suggestion was made to start with an experimental v0 version of the proposed EiffelArtifactDeployedEvent (see #322). If we decide to do this there are few practical matters that aren't obvious and therefore should be documented:

• What's the first version of an experimental event? 0.0.1 or 0.1.0 or something else?
• Are we allowed to make any kind of backwards incompatible changes within the v0 line? If so, how should SDKs handle the event versions? For normal non-experimental versions we can use the same data type for all event versions that share a major version because changes will always be forward compatible, but if that no longer is true we can't assume that a particular data type can be used for both, say, 0.1.0 and 0.2.0 of en event.
• ...

Motivation

If we're going to introduce such a concept we should document how it should be used so that we can be consistent. Documenting things also forces us to really think things through.

Exemplification

As a maintainer of eiffelevents-sdk-go I'd like to know if and how the experimental events will affect how I generate the SDK, or how the SDK can be used by clients.

Benefits

Less ambiguity.

Possible Drawbacks

None.

[e-backmark-ericsson]

Notes from community meeting on Dec 8th.

Reference info
Semver states that pre 1.x versions should be allowed to change anything: https://semver.org/#spec-item-4

What should the first version of an experimental event type be? 0.0.1 or 0.1.0?

• We decided it should be 0.1.0, in line with https://semver.org/#how-should-i-deal-with-revisions-in-the-0yz-initial-development-phase

What about links to experimental event types?

• It should be ok to link to an experimental event type from an existing event type. Minor version stepping as usual when adding such a link type, with a note saying "This is a link to an experimental event type which might be removed in a later version".
• If removing such a link to an experimental event type we should step the major version as usual.