Document multiple images in dox

Altering the README file to properly describe
how dox works with multiple images.

This includes how / where to specify them and how
dox uses them (executes the same prep/command code
on each image).

This is to better reflect behavior w/ new multiple
image patch.

Change-Id: Iba45177e6989c30d12d0b823956b2e5e07206176