README.md: document kola test writing
README.md: document kola test writing

glevand/coreos--mantle,mjg59/mantle,glevand/coreos--mantle,dm0-/mantle,mischief/mantle,quantum/coreos-mantle,quantum/coreos-mantle,marineam/mantle,marineam/mantle,mjg59/mantle,mischief/mantle,dm0-/mantle