refine README (#1355)

* refine README

fluid/faster_rcnn/README.md

@@ -57,6 +57,7 @@ After data preparation, one can start the training step by:
     sh ./pretrained/download.sh
 
 Set `pretrained_model` to load pre-trained model. In addition, this parameter is used to load trained model when finetuning as well.
+Please make sure that pretrained_model is downloaded and loaded correctly, otherwise, the loss may be NAN during training.
 
 **Install the [cocoapi](https://github.com/cocodataset/cocoapi):**
 

fluid/faster_rcnn/README_cn.md

@@ -57,6 +57,7 @@ Faster RCNN 目标检测模型
     sh ./pretrained/download.sh
 
 通过初始化`pretrained_model` 加载预训练模型。同时在参数微调时也采用该设置加载已训练模型。
+请在训练前确认预训练模型下载与加载正确，否则训练过程中损失可能会出现NAN。
 
 **安装[cocoapi](https://github.com/cocodataset/cocoapi)：**