docs(knowing app): add README for face detect

APP_Framework/Applications/knowing_app/face_detect/README.md

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+# Face detection demo
+
+### A face object detection task demo. Running MobileNet-yolo on K210-based edge devices.
+
+---
+
+## Training
+
+kmodel from [GitHub](https://github.com/kendryte/kendryte-standalone-demo/blob/develop/face_detect/detect.kmodel).
+
+## Deployment
+
+### compile and burn
+
+Use `(scons --)menuconfig` in bsp folder *(Ubiquitous/RT_Thread/bsp/k210)*, open:
+
+- More Drivers --> ov2640 driver
+- Board Drivers Config --> Enable LCD on SPI0
+- Board Drivers Config --> Enable SDCARD (spi1(ss0))
+- Board Drivers Config --> Enable DVP(camera)
+- RT-Thread Components --> POSIX layer and C standard library --> Enable pthreads APIs
+- APP_Framework --> Framework --> support knowing framework --> kpu model postprocessing --> yolov2 region layer
+- APP_Framework --> Applications --> knowing app --> enable apps/face detect
+
+`scons -j(n)` to compile and burn in by *kflash*.
+
+### json config and kmodel
+
+Copy json config for deployment o SD card */kmodel*. Example config file is *detect.json* in this directory. Copy final kmodel to SD card */kmodel* either.
+
+---
+
+## Run
+
+In serial terminal, `face_detect` to start a detection thread, `face_detect_delete` to stop it. Detection results can be found in output.