Create README.md (#798)

assets/demo/README.md

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+## :star2: Why OpenIM
+
+**🔍 Function screenshot display**
+
+<div align="center">
+
+
+|             multiple message              |               Efficient meetings                |
+| :---------------------------------------: | :---------------------------------------------: |
+| ![multiple-message](./multiple-message.png) | ![efficient-meetings](./efficient-meetings.png) |
+|      **One-to-one and group chats**       |     **Special features - Custom messages**      |
+|      ![group-chat](./group-chat.png)      |   ![special-function](./special-function.md)    |
+
+</div>