[DOC]Updated readmea with annotation info

Annotation info on the order of coords in the array

README.md

@@ -28,6 +28,7 @@ print(dataset.features)
 {
     'name':         Value(dtype='string', id=None),
     'annotations':  Array2D(shape=(None, 4), dtype='int32', id=None),
+    # ["xmin", "ymin", "xmax", "ymax"]
     'target':       Value(dtype='string', id=None),
     'image':        Image(decode=True, id=None)
 }