Well-written project documentation clarifies intent, documents decisions and results, and allows project managers to assess project progress (and report it, as necessary, to project stakeholders) at e
A product requirements document, or PRD: Read "Product Requirements Documents, Downsized" for assistance in creating this document.
https://www.atlassian.com/agile/product-management/requirements
User stories/scenarios and acceptance criteria: Review "Agile Requirements Snail: Feature to User Story to Scenario" for help in creating this document.
http://itsadeliverything.com/agile-requirements-snail-feature-to-user-story-to-scenario