fix ambiguity in the explanation of bounding box coordinates convention

README.md

@@ -45,7 +45,8 @@ ChainerCV follows the following conventions.
 + Channels of a color image are ordered as RGB. The shape of an image-array is CHW (channel, height, width).
 + Axis-dependent values follow row-column order.
   + Shape of an image is always represented as `(height, width)`.
-  + Coordinates of bounding boxes and keypoints are ordered as `(y, x)`.
+  + Coordinates of bounding boxes are ordered as `(y_min, x_min, y_max, x_max)`. `(y_min, x_min)` and `(y_max, x_max)` are coordinates of the top left and the bottom right vertices of a bounding box respectively.
+  + Coordinates of keypoints are ordered as `(y, x)`.
 
 ## Models
 Currently, ChainerCV supports networks for object detection and semantic segmentation.