add details to ubuntu packaging guide about keys on a headless server
add details to ubuntu packaging guide about keys on a headless server

fxtentacle/tilemill,MappingKat/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,tizzybec/tilemill,Zhao-Qi/tilemill,fxtentacle/tilemill,florianf/tileoven,mbrukman/tilemill,nyimbi/tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,MappingKat/tilemill,fxtentacle/tilemill,tizzybec/tilemill,florianf/tileoven,Teino1978-Corp/Teino1978-Corp-tilemill,paulovieira/tilemill-clima,fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,nyimbi/tilemill,paulovieira/tilemill-clima,tilemill-project/tilemill,mbrukman/tilemill,paulovieira/tilemill-clima,mbrukman/tilemill,mbrukman/tilemill,tilemill-project/tilemill,tizzybec/tilemill,tizzybec/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Zhao-Qi/tilemill,tilemill-project/tilemill,MappingKat/tilemill,nyimbi/tilemill,tizzybec/tilemill,paulovieira/tilemill-clima,Zhao-Qi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,nyimbi/tilemill,MappingKat/tilemill,fxtentacle/tilemill,florianf/tileoven,Zhao-Qi/tilemill,MappingKat/tilemill,nyimbi/tilemill,mbrukman/tilemill