add note about syncing manual to main README
add note about syncing manual to main README

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