Updates README.md to clarify writing middleware
Updates README.md to clarify writing middleware

Clarify that the `on_complete` handler requires an argument to yield up the `response_env` if they would like to affect the response.
davoclavo/faraday,mauricionr/faraday,stayhero/faraday,prcongithub/faraday,jrafanie/faraday,mehulkar/faraday,hstrowd/faraday,ru-faraon/Faraday-is-an-HTTP-client,engineyard/faraday,zhaoxl/faraday,hstrowd/faraday,apepper/faraday,daviswahl/faraday,jhass/faraday,ekampp/faraday,prcongithub/faraday,grosser/faraday,hrysd/faraday,pvdb/faraday,zhaoxl/faraday,luvtechno/faraday,santiagodoldan/faraday,davidcornu/faraday,iamjarvo/faraday,grosser/faraday,daviswahl/faraday,engineyard/faraday,zanker/faraday,O-I/faraday,jhass/faraday,santiagodoldan/faraday,coinbase/faraday,mehulkar/faraday,mauricionr/faraday,flower-pot/faraday,ngpestelos/faraday,ekampp/faraday,customink/faraday,luvtechno/faraday,nevans/faraday,davoclavo/faraday,coinbase/faraday,customink/faraday,lostisland/faraday,pvdb/faraday,stayhero/faraday,spyofchina/faraday,apepper/faraday,zanker/faraday,nevans/faraday,O-I/faraday,spyofchina/faraday,jrafanie/faraday,AvaelKross/faraday,davidcornu/faraday,ngpestelos/faraday,iamjarvo/faraday,flower-pot/faraday,lostisland/faraday,hrysd/faraday,AvaelKross/faraday,ru-faraon/Faraday-is-an-HTTP-client