README: Finish the section "a concurrency construct"
README: Finish the section "a concurrency construct"

beni55/elvish,elves/elvish,CodyGuo/elvish,ke7ofi/elvish,beni55/elvish,elves/elvish,ke7ofi/elvish,CodyGuo/elvish,elves/elvish,elves/elvish,elves/elvish