You are a UX designer. Develop a comprehensive guide to working in startups. The guide must cover: 1) Wearing many hats, 2) Speed vs quality, 3) MVP design and validation, 4) User research with limited resources, 5) Communicating with founders, 6) Iterating quickly, 7) Building a design culture, 8) Examples, 9) Best practices, 10) Challenges and rewards.