README: add blurb on async testing
README: add blurb on async testing

Fixes #132.
ArthurKK/Quick,Quick/Quick,dgdosen/Quick,jeffh/Quick,paulyoung/Quick,dgdosen/Quick,github/Quick,hwsyy/Quick,phatblat/Quick,nchatharoo/Quick,DrewKiino/Quick,hwsyy/Quick,wjszf/Quick,jondwillis/Quick,marciok/Quick,mrackwitz/Quick,phatblat/Quick,pedrovereza/Quick,Quick/Quick,pengleelove/Quick,jasonchaffee/Quick,jeffh/Quick,romankl/Quick,ashfurrow/Quick,DrewKiino/Quick,BlessNeo/Quick,jeffh/Quick,romankl/Quick,jessesquires/Quick,dcaunt/Quick,orta/Quick,mokagio/Quick,hanangellove/Quick,Quick/Quick,paulyoung/Quick,marciok/Quick,mrackwitz/Quick,wjszf/Quick,NatashaTheRobot/Quick,DanielAsher/Quick,ArthurKK/Quick,ichu501/Quick,cerberillo/Quick,ashfurrow/Quick,ikesyo/Quick,msdgwzhy6/Quick,mokagio/Quick,zhakui/Quick,phatblat/Quick,ikesyo/Quick,mokagio/Quick,jahilliard/Quick,ichu501/Quick,marciok/Quick,jondwillis/Quick,NatashaTheRobot/Quick,modocache/personal-fork-of-Quick,phatblat/Quick,pcantrell/Quick,hanangellove/Quick,cerberillo/Quick,joemasilotti/Quick,jasonchaffee/Quick,pedrovereza/Quick,DanielAsher/Quick,msdgwzhy6/Quick,jahilliard/Quick,luzefeng/Quick,dcaunt/Quick,Sweefties/Quick,pcantrell/Quick,BlessNeo/Quick,dgdosen/Quick,paulyoung/Quick,zhakui/Quick,nchatharoo/Quick,Quick/Quick,jasonchaffee/Quick,fullstackclub/Quick,jessesquires/Quick,fullstackclub/Quick,joemasilotti/Quick,luzefeng/Quick,github/Quick,orta/Quick,ikesyo/Quick,modocache/personal-fork-of-Quick,Sweefties/Quick,pengleelove/Quick