Add a warning in images.html about text-align

Add a warning in images.html about text-align

People normally center images using text-align. However, the `.img-responsive`
class changes the display type to block. As a result you can no longer align the
image with text-align. You must instead do it with margin: auto. There was no
note about this on the web page, and no mention about setting the display to
block. Users were left on a frustrating journey to figure out why they can't center
images anymore!