Update README
Update README

Emphasizes eikeon's original vision a bit better, shows off IIIF
features slightly higher in the docs, and pushes limitations to the
bottom.  Nobody wants the negative stuff on top, right?

uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server,uoregon-libraries/rais-image-server