add readme to pipeline paddledetecton models

python/examples/pipeline/PaddleDetection/faster_rcnn/README.md

+# Faster RCNN model on Pipeline Paddle Serving
+
+### Get The Faster RCNN Model
+```
+wget --no-check-certificate https://paddle-serving.bj.bcebos.com/pddet_demo/2.0/faster_rcnn_r50_fpn_1x_coco.tar
+```
+
+### Start the service
+```
+tar xf faster_rcnn_r50_fpn_1x_coco.tar
+python web_service.py
+```
+
+### Perform prediction
+
+```
+python pipeline_http_client.py
+```

python/examples/pipeline/PaddleDetection/ppyolo_mbv3/README.md

+# PPYOLO model on Pipeline Paddle Serving
+
+([简体中文](./README_CN.md)|English)
+
+### Get Model
+```
+wget --no-check-certificate https://paddle-serving.bj.bcebos.com/pddet_demo/2.0/ppyolo_mbv3_large_coco.tar
+```
+
+### Start the service
+```
+tar xf ppyolo_mbv3_large_coco.tar
+python web_service.py
+```
+
+### Perform prediction
+```
+python pipeline_http_client.py
+```
+

python/examples/pipeline/PaddleDetection/yolov3/README.md

+# YOLOv3 model on Pipeline Paddle Serving
+
+([简体中文](./README_CN.md)|English)
+
+### Get Model
+```
+wget --no-check-certificate https://paddle-serving.bj.bcebos.com/pddet_demo/2.0/yolov3_darknet53_270e_coco.tar
+```
+
+### Start the service
+```
+tar xf yolov3_darknet53_270e_coco.tar
+python web_service.py
+```
+
+### Perform prediction
+```
+python pipeline_http_client.py
+```
+