Follow the lead of the changes in I5e1bf85 for the at-ease library to
make README.md play nicer with Doxygen's markdown processor:

* Images used as the text of a link aren't rendered by Doxygen's
  markdown processor. Move the image source to a named link to make the
  output look a little nicer.
* Doxygen isn't recognizing "```" as a fenced code block delimiter (even
  though it is documented as an alternate syntax [0]) so use 4 spaces to
  mark code blocks instead.


[0]: http://michelf.com/projects/php-markdown/extra/#fenced-code-blocks

Change-Id: Ifc3b3f1d4f512dd12edd715ca2c2a5682c63a4b0