docs: document working with two build directories
docs: document working with two build directories

apple/swift,roambotics/swift,roambotics/swift,JGiola/swift,roambotics/swift,glessard/swift,ahoppen/swift,JGiola/swift,benlangmuir/swift,JGiola/swift,ahoppen/swift,benlangmuir/swift,roambotics/swift,glessard/swift,glessard/swift,glessard/swift,apple/swift,ahoppen/swift,benlangmuir/swift,apple/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,apple/swift,ahoppen/swift,benlangmuir/swift,apple/swift,apple/swift,ahoppen/swift,glessard/swift,roambotics/swift,JGiola/swift,JGiola/swift,ahoppen/swift