Without having actually tested it yet, I have just a few suggestions:
tpl_product_info_display.php looks fine, but it would be good to include a comment in the top section about the purpose and date of the specific change.
Everyone already has a modified stylesheet, and the stylesheet.css would erase their changes if carelessly uploaded.
It would be better to put only the modified stylesheet sections in a file named stylesheet_additional_images.css or something, so they could safely and easily be added/merged with the existing.
It would be helpful to put the date, title, version and other "header" info at the top of the readme. I would also suggest putting the whole CSS block there for easy reference, as it is not very large.
For consistency with other tags, it might be better to name the wrapper #additionalImagesWrapper, or perhaps #productImagesWrapper since it is for all the product images, not just additional ones.
/beginning of additional images code/
#additionalImageWrapper {
float: left;
width: 120px
}
#productAdditionalImages .additionalImages a {
float: left;
margin-left: 10px;
}
/end of additional images code/
One typo - "root/includes/templates/your_template/templates/tpl_product_display_info.php contains "
Good work so far, and I will test it when I have time tonight - I don't expect any difficulties.