add model url

MODEL_ZOO.md

@@ -75,9 +75,9 @@ We released RPN, Faster R-CNN and Mask R-CNN models in the first version. More m
 
 | Backbone | Style   | Lr schd | Mem (GB) | Train time (s/iter) | Inf time (fps) | box AP | Download |
 |:--------:|:-------:|:-------:|:--------:|:-------------------:|:--------------:|:------:|:--------:|
-| R-50-FPN | caffe   | 1x      | 5.8      | 0.459               | 9.4            |        | -        |
-| R-50-FPN | pytorch | 1x      | 6.2      | 0.487               | 9.1            |        |          |
-| R-50-FPN | pytorch | 2x      | 6.2      | 0.487               | 9.1            |        |          |
+| R-50-FPN | caffe   | 1x      | 6.7      | 0.468               | 9.4            | 35.8   | -        |
+| R-50-FPN | pytorch | 1x      | 6.9      | 0.496               | 9.1            | 35.6   | [model](https://s3.ap-northeast-2.amazonaws.com/open-mmlab/mmdetection/models/retinanet_r50_fpn_1x_20181125-3d3c2142.pth) |
+| R-50-FPN | pytorch | 2x      | 6.9      | 0.496               | 9.1            | 36.5   | [model](https://s3.ap-northeast-2.amazonaws.com/open-mmlab/mmdetection/models/retinanet_r50_fpn_2x_20181125-e0dbec97.pth) |
 
 
 ## Comparison with Detectron