Merge pull request #146 from WuHaobo/en_docs

en docs of the extensions

docs/en/extension/multi_machine_training.md

+# Distributed Training
+
+Distributed deep neural networks training is highly efficient in PaddlePaddle. 
+And it is one of the PaddlePaddle's core advantage technologies.
+On image classification tasks, distributed training can achieve almost linear acceleration ratio.
+[Fleet](https://github.com/PaddlePaddle/Fleet) is High-Level API for distributed training in PaddlePaddle. 
+By using Fleet, a user can shift from local machine paddlepaddle code to distributed code easily.
+In order to support both single-machine training and multi-machine training,
+[PaddleClas](https://github.com/PaddlePaddle/PaddleClas) uses the Fleet API interface.
+For more information about distributed training,
+please refer to [Fleet API documentation](https://github.com/PaddlePaddle/Fleet/blob/develop/README.md).
+

docs/en/extension/paddle_hub.md

+# Paddle Hub
+
+[PaddleHub](https://github.com/PaddlePaddle/PaddleHub) is a pre-trained model application tool for PaddlePaddle.
+Developers can conveniently use the high-quality pre-trained model combined with Fine-tune API to quickly complete the whole process from model migration to deployment.
+All the pre-trained models of [PaddleClas](https://github.com/PaddlePaddle/PaddleClas) have been collected by PaddleHub.
+For further details, please refer to [PaddleHub website](https://www.paddlepaddle.org.cn/hub).