Flush WordPress rewrite rules when the MD5 sum of the add post type file changes.

gistfile1.php

// I put this code in the file where I add my custom post types / taxonomies. 
// Flush rewrite rules on change of this file.
if( get_option('custom_pt_rules') != md5_file( __FILE__ ) )
{
    add_action( 'init', function()
            {
                global $wp_rewrite;
                $wp_rewrite->flush_rules();
            });
    update_option('custom_pt_rules', md5_file( __FILE__ ) );
}