add RISEN framework to contributing and readme

[earlduque]

I could've sworn I did this, but I don't see it anywhere. We should add the RIESN framework to our documentation to make it more obvious what we're looking for:

Need some guidance? The RISEN framework helps structure tasks for generative AI into actionable steps:
R — Role: Define the AI's role. (e.g., "Act as an agile coach.")
I — Instructions: Provide clear instructions. (e.g., "Write a guide on crafting a good agile development story.")
S — Steps: Outline specific steps. (e.g., "Start with a user persona, describe the user's goal, list the tasks to achieve the goal, and define the acceptance criteria.")
E — End goal: State the goal and audience. (e.g., "Help team members understand how to write clear and effective agile stories.")
N — Narrowing: Add constraints. (e.g., "Max 800 words, avoid technical jargon, use practical examples.")