From your reading response document, share your most interesting question with the class. This is probably from your "questions for the author" section or the "any other questions/comments" section. Participation (at the level of posting a question) is required for credit, but it shouldn't be hard at all assuming you've already prepared your response document.
Feel free to try to answer someone else's question, elaborate on the question, or just "like" it.
Can't change a rubric once you've started using it.