Add read more to the_content

You could add a link manually like:

<a href="https://wordpress.stackexchange.com/questions/88976/<?php echo get_permalink($post->ID) ?>">Read More...</a>

If you want to add the link at the end of the content you could do:

return implode(' ', array_slice($words, 0, $word_limit)) . '<a href="'.get_permalink($post->ID).'">Read More...</a>';