Add Read More text after the excerpt (Only Excerpts which are the hand-crafted summaries of your content ).

functions.php

<?php

// Add "Read More" link to excerpts only (Excerpts which are the hand-crafted summaries of your content ))
add_filter('get_the_excerpt', 'custom_excerpt_read_more_link');
function custom_excerpt_read_more_link($excerpt) {
    $excerpt_more = '';
    $read_more_text    = 'Read More &raquo;';
    if ( has_excerpt() && ! is_attachment() && get_post_type() == 'post' ) {
        $excerpt_more = '<p class="read-more" ><a href="' . esc_url( get_permalink() ) . '"> '. $read_more_text . '</a></p>';
    }
    return $excerpt . $excerpt_more;
}