Update README.md

Author: jwyang

README.md

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 Pytorch implementation of Feature Pyramid Network (FPN) for Object Detection
 
 ## Introduction
+
+This project inherit the property of our [pytorch implementation of faster r-cnn](https://github.com/jwyang/faster-rcnn.pytorch). Hence, it also has the following unique features:
+
+* **It is pure Pytorch code**. We convert all the numpy implementations to pytorch.
+
+* **It supports trainig batchsize > 1**. We revise all the layers, including dataloader, rpn, roi-pooling, etc., to train with multiple images at each iteration.
+
+* **It supports multiple GPUs**. We use a multiple GPU wrapper (nn.DataParallel here) to make it flexible to use one or more GPUs, as a merit of the above two features.
+
+* **It supports three pooling methods**. We integrate three pooling methods: roi pooing, roi align and roi crop. Besides, we convert them to support multi-image batch training.
+
+## Progress
+
+Performance testing, finished soon!