Rewrite rule for not having to delete robots.txt

This is just a little trick that could be added to the docs.

For multisites in which you cannot delete the robots.txt file because you only want to use this module in some sites you can add this rewrite rule to virtual host config for the sites in which you are using the module

RewriteRule /robots.txt /index.php?q=robots.txt [L,QSA]