Revisit README statement that Assets-Masters will not contain any files not related to images

[stephengtuggy]

This does not appear to be the case any longer. Assets-Masters now holds many files that are not directly related to images or textures. We probably need to change the text here, but I would be fine with doing that in a followup PR if need be.

Originally posted by @stephengtuggy in #93 (comment)