add cover image for sotopia-pi (#191)

README.md

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+![sotopia-pi](assets/sotopia-pi.png)
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 # Sotopia-π: Interactive Learning of Socially Intelligent Language Agents
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   [![Python 3.11](https://img.shields.io/badge/python-3.11-blue.svg)](https://www.python.org/downloads/release/python-3109/)
   [![pre-commit](https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit&logoColor=white)](https://pre-commit.com/)
   <a href="https://github.com/psf/black"><img alt="Code style: black" src="https://img.shields.io/badge/code%20style-black-000000.svg"></a>
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   ## 📢 Release
   [03/14] 🎆We released our paper on [arXiv](https://arxiv.org/abs/2403.08715) on 3/14 PI day and the paper was reported by AK on twitter ([here](https://twitter.com/_akhaliq/status/1768118035770974262)).
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   [03/07] 🔥We released our model checkpoints (BC, SR, BC+SR) on huggingface ([BC model](https://huggingface.co/cmu-lti/sotopia-pi-mistral-7b-BC), [SR model](https://huggingface.co/cmu-lti/sotopia-pi-mistral-7b-SR), [BC+SR model](https://huggingface.co/cmu-lti/sotopia-pi-mistral-7b-BC_SR)).
 
   [03/04] 📊We released our social converation data on huggingface ([here](https://huggingface.co/datasets/cmu-lti/sotopia-pi/tree/main)).