update docstring (#1625)

* Update GETTING_STARTED.md

Found out that this method is required. The torch.nn.Module doesn't contain this method thus you need to implement it

* update docstring 

see https://github.com/open-mmlab/mmdetection/issues/1582

* correct for flake8

* flake8: trailing space has removed

mmdet/datasets/pipelines/transforms.py

@@ -581,8 +581,10 @@ class MinIoURandomCrop(object):
     selected from min_ious.
 
     Args:
-        min_ious (tuple): minimum IoU threshold
-        crop_size (tuple): Expected size after cropping, (h, w).
+        min_ious (tuple): minimum IoU threshold for all intersections with
+        bounding boxes
+        min_crop_size (float): minimum crop's size (i.e. h,w := a*h, a*w,
+        where a >= min_crop_size).
     """
 
     def __init__(self, min_ious=(0.1, 0.3, 0.5, 0.7, 0.9), min_crop_size=0.3):