add a README for training object detection models (#1612)

references/detection/README.md

+# Object detection reference training scripts
+
+This folder contains reference training scripts for object detection.
+They serve as a log of how to train specific models, as provide baseline
+training and evaluation scripts to quickly bootstrap research.
+
+Except otherwise noted, all models have been trained on 8x V100 GPUs.
+
+### Faster R-CNN
+```
+python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
+    --dataset coco --model fasterrcnn_resnet50_fpn --epochs 26\
+    --lr-steps 16 22 --aspect-ratio-group-factor 3
+```
+
+
+### Mask R-CNN
+```
+python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
+    --dataset coco --model maskrcnn_resnet50_fpn --epochs 26\
+    --lr-steps 16 22 --aspect-ratio-group-factor 3
+```
+
+
+### Keypoint R-CNN
+```
+python -m torch.distributed.launch --nproc_per_node=8 --use_env train.py\
+    --dataset coco_kp --model keypointrcnn_resnet50_fpn --epochs 46\
+    --lr-steps 36 43 --aspect-ratio-group-factor 3
+```
+