OpenCV dimensionality inconsistencies need caution callout about image-processing HOT 1 CLOSED

Added the following callout to the top of 03-opencv-images:

Coordinate and color channel order

CAUTION: it is vital to remember the order of the coordinates and color
channels when dealing with OpenCV images as NumPy arrays. If we are
manipulating or accessing an image array directly, we specifiy the y
coordinate first, then the x. Further, the first channel stored is the
blue channel, followed by the green, and then the red.

{: .callout}

from image-processing.