Config.php contains the information needed to access your database. Thus, it needs to be retained when upgrading (which is why the file distributed is config.php.txt to prevent overwriting).

If you delete your config.php for some reason, here's what you can do:
1) Perform a new install at a different location.
2) Copy the config.php from that new install over to the original.
3) Open the config.php and edit the URL and file path to reflect the proper location.

Alternately, advanced users can re-create a config.php by hand editing based on this (5.0 series) template:

<?php
$dbtype = 'mysqli';
if (!function_exists('mysqli_connect')) $dbtype == 'mysql';
require_once 'databases/'.$dbtype.'.php';
$dbuser = 'root';
$dbpass = '';
$dbhost = 'localhost';
$databasename = 'wsnlinks';
$adb = new db($dbhost, $dbuser, $dbpass, $databasename);
if ($adb->dbh) $connection = $adb->dbh;
$prefix = 'wsnlinks_';

 $dirurl = 'http://localhost/50/wsnlinks'; 
 $uploadpath = '/var/www/50/wsnlinks/attachments/'; 
 $rejectspiderlevel = ''; 
 $rejectguestlevel = ''; 
 $ftphost = ''; 
 $ftpuser = ''; 
 $ftppass = ''; 
 $ftpdir = ''; 
 ?>