Fix image links. Replace the thumb key wiring diagram.

Image links in the original `README.org` work on GitHub, but did not work for me when I tried exporting `README.org` to HTML using Org-mode (version 8.2.5g).

Also, GitHub's org-to-html converter (org-ruby?) insisted on interpreting the description of the link to `case.svg` as an inline image which then failed to get inlined. So I added some text to fool it.