Update README

[lukehoban]

• [x] Overview of tool
• [x] Warnings that this is an experiment, and should be used with extreme caution!
• [x] Pointer to the website
• [x] A couple of demos walkthroughs
• [x] Instructions for running with npx
• [ ] API of the PulumiAI class
• [ ] Node 18 requirement
• [x] Need credentials for any cloud that you plan to request infrastructure
• [x] Need OpenAI credentials OPENAI_API_KEY
• [ ] Need Pulumi packages installed for any providers you plan to request infrastructure
• [x] Describe how it works (OpenAI GPT API + Prompt Engineering + future generative search)

[pulumi-bot]

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