Example
Adds a rights element to all Atom feed entries.

Uses the post entry’s publication year and the blog name for copyright.

add_action( 'atom_entry', 'example_1483343_atom_entry', 10 );

function example_1483343_atom_entry() {
	printf( '<rights type="text">&copy; %1$s %2$s</rights>', get_the_date( 'Y' ), get_bloginfo ( 'name' ) );
}