Act as a Python developer with 3+ years of experience in game development. Create a step-by-step guide for building a rock-paper-scissors game in Python. Include the following details: [1] Explain how to use `random.choice()` for the computer's move, [2] Describe how to validate user input (only 'rock', 'paper', or 'scissors'), and [3] Show how to determine the winner using conditional statements. Ensure the code is beginner-friendly with comments for clarity. Also, suggest [optional: adding a score tracker] or [optional: extending the game to best-of-5 rounds] for advanced learners. Provide the full code snippet at the end.