When reading about "Stahl House", Henry finds an interesting image of the house where some parts are annotated. Henry discovers that annotations are possible and feels invited to add more. He adds some annotations for testing and is able to quickly remove his own tests without affecting the existign ones. Finally, he annotates a piece of furniture to indicate it was the "tulip chair" by Eero Saarinen. Later, when going back to the article Henry feels he has made a good contribution.
Annotations coulde be added by added by active readers and editors alike. Letting them know that this adds value is important if we want to encourage them.