add document for detection demo

python/paddle_fl/paddle_fl/examples/detection_demo/README.md

+# Example of a detection model training with FedAvg
+
+This document introduce how to start a detection model training in PaddleFL with our pre-defined program. Now we only provide faster_rcnn, more models will be updated. 
+
+### Dependencies
+
+- paddlepaddle>=1.8
+- paddle_fl>=1.0
+
+Please use pip which has paddlepaddle installed
+
+```sh
+pip install paddle_fl
+``` 
+
+
+### How to Run
+
+#### Download the dataset
+
+```sh
+# download and unzip the dataset
+sh download.sh
+```
+
+#### Start training 
+
+Before training, please modify the following paths according to your environment.
+
+```python
+
+# In run.sh, change the path to you PaddleDetection
+export PYTHONPATH=/path/to/PaddleDetection
+
+# In fl_train.py, change the path to your fl_fruit dataset that is downloaded in download.sh. 
+# Note, the path should be absolute path rather than relatice path. Otherwise, error will be raised. 
+
+#line 41
+self.root_path = '/path/to/your/fl_fruit'
+#line 42
+self.anno_path = '/path/to/your/fl_fruit/train' + str(trainer_id) + '.txt'
+# line 44
+self.image_dir = '/path/to/your/fl_fruit/JPEGImages'
+# line 57
+label_list='/path/to/your/fl_fruit/label_list.txt'
+
+```
+
+After modifying the path, you can run the following shell directly.
+
+```sh
+
+sh run.sh
+```