README.md: Link to GitHub releases for the most-recent compiled version

This way we don't need to remember to push to
files.software-carpentry.org.  The linked page has the message from
the latest release and a link to the exectuable, when it used to be
just the exectuable.  A quick search didn't turn up a way to link
directly to the latest binary.  I'm not particularly worried about the
additional click though, and folks working up installation
instructions for a given workshop should probably be linking to an
explicit version anyway.