Update README.md

Author: seratch

README.md

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 # ChatGPT in Slack
 
+> **:warning: The official Slack integration from OpenAI is now available! You may want to try it first: https://my.slack.com/marketplace/A097V82EGG2-chatgpt**
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 Introducing a transformative app for Slack users, specifically designed to enhance your communication with [ChatGPT](https://openai.com/blog/chatgpt)!
 This app enables seamless interaction with ChatGPT via Slack channels, optimizing your planning and writing processes by leveraging AI technology.