Update readme with new middleware signature.
Update readme with new middleware signature.

jrallison/go-workers,emailhunter/workers,outrightmental/go-workers,zodsoft/go-workers,Scalingo/go-workers,timehop/go-workers,khaf/go-workers,chrisnordqvist/go-workers,trigrass2/go-workers,LubyRuffy/go-workers