add docs/en/extension/paddle_hub.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).