update README

ppgan/metric/README.md

@@ -8,3 +8,12 @@ wget https://paddlegan.bj.bcebos.com/InceptionV3.pdparams
 ```
 python test_fid_score.py --image_data_path1 /path/to/dataset1 --image_data_path2 /path/to/dataset2 --inference_model ./InceptionV3.pdparams
 ```
+
+### Inception-V3 weights converted from torchvision
+
+Download: https://aistudio.baidu.com/aistudio/datasetdetail/51890
+
+This model weights file is converted from official torchvision inception-v3 model. And both BigGAN and StarGAN-v2 is using it to calculate FID score.
+
+Note that this model weights is different from above one (which is converted from tensorflow unofficial version)
+