add notes about cross-compiling to DEVELOPER_README
add notes about cross-compiling to DEVELOPER_README

Lasercake/Lasercake,elidupree/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,idupree/Lasercake,elidupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,idupree/Lasercake,elidupree/Lasercake,Lasercake/Lasercake,idupree/Lasercake,elidupree/Lasercake,elidupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,elidupree/Lasercake