Patch to INSTALL adding note about web server needing to be able to resolve the site's URL

This patch adds a step (at #2) noting that the web server must be able to resolve the site's URL for taxonomy_image to work. If you are adding images to your terms, seeing them in the local filesystem, but not seeing them in the taxonomy term image list this could be the reason.

As an aside, I was surprised that taxonomy_image uses the sites URL instead of the local filesystem for building it's cache, but I expect this is done to work with content distribution networks or the like.