prevent dir listings

This rule permits urls like:

    /css/
    /js/
    /img/

to show a file listing (depending on apache setup of course). By default
this most likely is not what users want
Add recommendation to use h5bp's .htaccess file

Or other server config as appropriate.
Merge pull request #34 from FriendsOfCake/no-webroot-dir-listing

prevent dir listings