Take some of the less obvious options out of usage() and point folks to the
man page for them instead. This makes the output of 'duperemove --help' more
concise while also giving users a pointer to where they can read a lot more
about duperemove.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Update README

Signed-off-by: Mark Fasheh <mfasheh@suse.de>