function imnobby_append_to_content( $content ) {
 
 $tmphtml = "";
 
 if ( get_field('keywords') ){
 $tmphtml .= '<i class="small">Related Keywords: ' . get_field('keywords') . '</i>'; // "additional content";
 }
 
 $content .= $tmphtml;
 
 return $content;
}
add_filter( 'the_content', 'imnobby_append_to_content', 99);
Related Keywords: the_content function