README: Drop '**' from "Contributing" header
README: Drop '**' from "Contributing" header

These are from f4d6188f (Make it easier for CONTRIBUTORS to find
information, 2018-07-05, #474).  I dunno if they were intentional or
not, but I thought they might be footnote markers so I scrolled down
to the bottom of the README looking for a footnote.  Remove them to
save other people from that same confusion ;).

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

containers/image,containers/image