Removed link to addon page in README.md

The link was leading to a page that said : " This add-on has been
removed by its author.i: When I further investigated I also found a
message on the Mozmill Developpers Google Groups
(https://groups.google.com/forum/#!topic/mozmill-dev/02HwCHWQFRA)
mentionning the addon had been removed from addons.mozilla.oth since
there hadn't been any work done on it for a long time and suport for the
addon has already been dropped.