Add an ActivityStreams example in the spec

[pietercolpaert]

Recurring question is: how do we indicate something has been removed, created or updated? Activity Streams 2.0 does this however. Examples of AS2 applied to data management can be found in for example:

• https://www.eventnotifications.net/
• IIIF Change Discovery: https://iiif.io/api/discovery/1.0/