Update README.md

README.md

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 ## Fine-Grained Behavior Simulation with Role-Playing Large Language Model on Social Media
 
 We collect 78.6k fine-grained user behavior prediction QA data from 1866 users on social medias(including Twitter, Reddit and Zhihu)
-You can find the detailed user infomation in *platform_user_info.json* through the *user_index*. 
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+You can find the detailed user infomation in *platform_user_info.json* through the *user_index*. 
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+*history_index* means the behavior index in user's complete timelines, basicly, you should predict this behavior by the history. The history_index is 0 means it is the first behavior of the user.