Update README.md

TensorFlow2Paddle/README.md

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 **Notice：You can install PaddlePaddle and Tensorflow in different virtual environment since there's dependency conflict between PaddlePaddle and TensorFlow**
 
+### Usage
+**1. Model file: Tensorflow checkpoint directory**
+**2. input tensors' and output tensors' name**
+
 ### Demo: How to change tensorflow resnet_v1_50 pretrained model to PaddlePaddle model for inference
 #### 1. Get pretrained_model