Display update notification messages like ‘What’s New’

You can hook into in_plugin_update_message-{$file}.

For the above plugin, the according code looks as follows:

add_action('admin_menu', array($this,'admin_menu'), 11);
function admin_menu()
{
    global $pagenow;

    if( $pagenow == 'plugins.php' )
    {
        $hook = apply_filters('acf/get_info', 'hook');

        add_action( 'in_plugin_update_message-' . $hook, array($this, 'in_plugin_update_message'), 10, 2 );
    }
}

function in_plugin_update_message( $plugin_data, $r )
{
    $version = apply_filters('acf/get_info', 'version');
    $readme = wp_remote_fopen( 'http://plugins.svn.wordpress.org/advanced-custom-fields/trunk/readme.txt' );
    $regexp = '/== Changelog ==(.*)= ' . $version . ' =/sm';
    $o = '';

    if( !$readme )
    {
        return;
    }

    preg_match( $regexp, $readme, $matches );

    if( ! isset($matches[1]) )
    {
        return;
    }

    $changelog = explode('*', $matches[1]);
    array_shift( $changelog );

    if( !empty($changelog) )
    {
        $o .= '<div class="acf-plugin-update-info">';
        $o .= '<h3>' . __("What's new", 'acf') . '</h3>';
        $o .= '<ul>';

        foreach( $changelog as $item )
        {
            $item = explode("https://wordpress.stackexchange.com/questions/129392/http", $item);

            $o .= '<li>' . $item[0];

            if( isset($item[1]) )
            {
                $o .= '<a href="https://wordpress.stackexchange.com/questions/129392/http" . $item[1] . '" target="_blank">' . __("credits",'acf') . '</a>';
            }

            $o .= '</li>';


        }

        $o .= '</ul></div>';
    }

    echo $o;
}

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