readme: add link to coreos/bugs
readme: add link to coreos/bugs

coreos/ignition,ajeddeloh/ignition,dgonyeo/ignition,jonboulle/ignition,ajeddeloh/ignition,coreos/ignition,ajeddeloh/ignition,coreos/ignition,AlexNPavel/ignition,higebu/ignition,AlexNPavel/ignition,jonboulle/ignition,AlexNPavel/ignition,jonboulle/ignition,higebu/ignition,higebu/ignition,dgonyeo/ignition,dgonyeo/ignition