Add diffusion example to README (#20161)

Author: awaelchli

README.md

@@ -143,6 +143,7 @@ Explore various types of training possible with PyTorch Lightning. Pretrain and
 | [Text summarization](https://lightning.ai/lightning-ai/studios/text-summarization-with-pytorch-lightning)   | Finetune - text summarization (Hugging Face transformer model) | <a target="_blank" href="https://lightning.ai/lightning-ai/studios/text-summarization-with-pytorch-lightning"><img src="https://pl-bolts-doc-images.s3.us-east-2.amazonaws.com/app-2/studio-badge.svg" alt="Open In Studio"/></a> |   
 | [Audio generation](https://lightning.ai/lightning-ai/studios/finetune-a-personal-ai-music-generator)        | Finetune - audio generator (transformer model)                 | <a target="_blank" href="https://lightning.ai/lightning-ai/studios/finetune-a-personal-ai-music-generator"><img src="https://pl-bolts-doc-images.s3.us-east-2.amazonaws.com/app-2/studio-badge.svg" alt="Open In Studio"/></a> |   
 | [LLM finetuning](https://lightning.ai/lightning-ai/studios/finetune-an-llm-with-pytorch-lightning)          | Finetune - LLM (Meta Llama 3.1 8B)                | <a target="_blank" href="https://lightning.ai/lightning-ai/studios/finetune-an-llm-with-pytorch-lightning"><img src="https://pl-bolts-doc-images.s3.us-east-2.amazonaws.com/app-2/studio-badge.svg" alt="Open In Studio"/></a> | 
+| [Image generation](https://lightning.ai/lightning-ai/studios/train-a-diffusion-model-with-pytorch-lightning)          | Pretrain - Image generator (diffusion model)                | <a target="_blank" href="https://lightning.ai/lightning-ai/studios/train-a-diffusion-model-with-pytorch-lightning"><img src="https://pl-bolts-doc-images.s3.us-east-2.amazonaws.com/app-2/studio-badge.svg" alt="Open In Studio"/></a> | 
 
 ### Hello simple model