add busybox test instructions to README.md
add busybox test instructions to README.md

jbcrail/coreutils,dbrgn/coreutils,uutils/coreutils,kevgeniy/coreutils,wimh/coreutils,Arcterus/coreutils,uutils/coreutils,kwantam/coreutils,kevgeniy/coreutils,keunwoo/coreutils,keunwoo/coreutils,ebfe/coreutils,kwantam/coreutils,Heather/coreutils,dbrgn/coreutils,Arcterus/coreutils,wimh/coreutils,Heather/coreutils,tempbottle/coreutils,ebfe/coreutils,jbcrail/coreutils,tempbottle/coreutils,dbrgn/coreutils,uutils/coreutils