add multi_machine_training docs

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).
+