Enhance readme in vista3d notebook

[KumoLiu]

suggest including details about the model and outlining what users can expect from the notebook in the ReadMe. The notebook currently lacks information regarding the VISTA 3D model and the fine-tuning task. We recommend adding this content to the ReadMe.md file.

[KumoLiu]

Hi @heyufan1995, could you please help enhance the notebook regarding the feedback from internal test? Thanks.