[cherry-pick] update docs with cpu training (#2926)

Author: jerrywgz

PaddleCV/PaddleDetection/docs/GETTING_STARTED.md

@@ -20,13 +20,16 @@ python tools/train.py -c configs/faster_rcnn_r50_1x.yml
 
 ```bash
 export CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
+# or run on CPU with:
+# export CPU_NUM=8
 python tools/train.py -c configs/faster_rcnn_r50_1x.yml
 ```
 
 - Datasets is stored in `dataset/coco` by default (configurable).
 - Pretrained model is downloaded automatically and cached in `~/.cache/paddle/weights`.
 - Model checkpoints is saved in `output` by default (configurable).
 - To check out hyper parameters used, please refer to the config file.
+- RCNN models training on CPU is not supported on PaddlePaddle<=1.5.1 and will be fixed on later version.
 
 Alternating between training epoch and evaluation run is possible, simply pass
 in `--eval=True` to do so (tested with `SSD` detector on Pascal-VOC, not