doc: add DCO
doc: add DCO

trnubo/ignition,coreos/ignition,ajeddeloh/ignition,trnubo/ignition,higebu/ignition,AlexNPavel/ignition,AlexNPavel/ignition,higebu/ignition,mischief/ignition,dgonyeo/ignition,ajeddeloh/ignition,marineam/ignition,jonboulle/ignition,higebu/ignition,marineam/ignition,crawford/ignition,marineam/ignition,ajeddeloh/ignition,mischief/ignition,coreos/ignition,dgonyeo/ignition,dgonyeo/ignition,crawford/ignition,jonboulle/ignition,trnubo/ignition,mischief/ignition,AlexNPavel/ignition,coreos/ignition,jonboulle/ignition