add README

ppgan/metric/README.md

+English | [简体中文](./README.md)
+
+# Usage
+
+To compute the FID score between two datasets, where images of each dataset are contained in an individual folder:
+
+```
+python test_fid_score.py --image_data_path1 /path/to/dataset1 --image_data_path2 /path/to/dataset2 --inference_model /path/to/InceptionV3/inference_moel
+```