add README

ppgan/metric/README.md

@@ -4,6 +4,7 @@ English | [简体中文](./README.md)
 
 To compute the FID score between two datasets, where images of each dataset are contained in an individual folder:
 
+Download [InceptionV3](https://paddlegan.bj.bcebos.com/InceptionV3.pdparams) inference model, and then run
 ```
 python test_fid_score.py --image_data_path1 /path/to/dataset1 --image_data_path2 /path/to/dataset2 --inference_model /path/to/InceptionV3/inference_moel
 ```