add cornernet (#3492)

* add cornernet

* update model zoo

README.md

@@ -96,6 +96,7 @@ Supported methods:
 - [x] [GRoIE](configs/groie/README.md)
 - [x] [DetectoRS](configs/detectors/README.md)
 - [x] [Generalized Focal Loss](configs/gfl/README.md)
+- [x] [CornerNet](configs/cornernet/README.md)
 
 Some other methods are also supported in [projects using MMDetection](./docs/projects.md).
 

docs/model_zoo.md

@@ -144,6 +144,9 @@ Please refer to [DetectoRS](https://github.com/open-mmlab/mmdetection/blob/maste
 ### Generalized Focal Loss
 Please refer to [Generalized Focal Loss](https://github.com/open-mmlab/mmdetection/blob/master/configs/gfl) for details.
 
+### CornerNet
+Please refer to [CornerNet](https://github.com/open-mmlab/mmdetection/blob/master/configs/cornernet) for details.
+
 ### Other datasets
 
 We also benchmark some methods on [PASCAL VOC](https://github.com/open-mmlab/mmdetection/blob/master/configs/pascal_voc), [Cityscapes](https://github.com/open-mmlab/mmdetection/blob/master/configs/cityscapes) and [WIDER FACE](https://github.com/open-mmlab/mmdetection/blob/master/configs/wider_face).