add a reminder about multi-channel images

[tobyhodges]

In episode 4, lines 163-167, the learner is asked to create a blank canvas in three dimensions. Since it will have been a little while since the concept of multi-channel images was introduced, it would be helpful to include a (brief) reminder here, explaining why the third dimension is being used for the shape of this new canvas.

> First let's make an empty, black image with a size of 800x600 pixels:
> 
> ~~~
> # create the black canvas
> image = np.zeros(shape=(600, 800, 3), dtype="uint8")

[uschille]

Related to #103.