There are various ways to accomplish this, but I think it’s neatest and “most WordPress” to enqueue the fonts in your functions file, and then you can specifiy that they should load in the footer using the appropriate parameter:
function wpb_add_google_fonts() {
wp_enqueue_style( 'wpse-google-fonts', 'http://fonts.googleapis.com/css?family=Roboto', '', '', true );
}
add_action( 'wp_enqueue_scripts', 'wpse_add_google_fonts' );