update the README (#13186)

Author: stu1130

example/ssd/README.md

@@ -102,6 +102,7 @@ will open a window that will display the camera output together with the detecti
 with the detection threshold to get more or less detections.
 
 ### Train the model
+* Note that we recommend to use gluon-cv to train the model, please refer to [gluon-cv ssd](https://gluon-cv.mxnet.io/build/examples_detection/train_ssd_voc.html).
 This example only covers training on Pascal VOC dataset. Other datasets should
 be easily supported by adding subclass derived from class `Imdb` in `dataset/imdb.py`.
 See example of `dataset/pascal_voc.py` for details.