This week's book giveaways are in the Refactoring and Agile forums. We're giving away four copies each of Re-engineering Legacy Software and Docker in Action and have the authors on-line! See this thread and this one for details.
I think one section on design/architecture decisions is fine. These decisions have been taken to support the functional & non-functional requirements. As reason/rationale for taking the decision you would mention the functional or non-functional requirement.