docs(knowing framework): add README for YOLOv2 region layer

APP_Framework/Framework/knowing/kpu-postprocessing/yolov2/README.md

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+# KPU(K210) YOLOv2 region layer
+
+## Introduction
+
+KPU(k210) accelerate most of CNN network layers, but do not support some of operators of YOLOv2 region layer. Such layers and operators will run on MCU instead.  
+YOLOv2 region layer accept feature map(shape w\*h\*c) and return final detection boxes.
+
+## Usage
+
+Use `(scons --)menuconfig` in bsp folder *(Ubiquitous/RT_Thread/bsp/k210)*, open *APP_Framework --> Framework --> support knowing framework --> kpu model postprocessing --> yolov2 region layer*.