NEW：Add README file

Author: flame.yu

README.md

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+# Arduino Core for RoboMaster i.MX RT Boards
+
+[中文](README_CN.md)
+
+> Thanks for this work:
+> https://github.com/Seeed-Studio/ArduinoCore-imxrt
+
+This project provides an Arduino compatible layer for the NXP I.MXRT series chip development board, which now supports:
+
+- RM EXPANSION MODULE
+
+For more information see Read the doc

README_CN.md

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+# Arduino Core for RoboMaster i.MX RT Boards
+
+> 感谢该项目所作的工作：
+> https://github.com/Seeed-Studio/ArduinoCore-imxrt
+
+本项目为 NXP I.MXRT 系列芯片开发板提供了 Arduino 兼容层，支持：
+
+- 机甲大师通用拓展模块
+
+更多资料详见 Read the doc