precise that the plugin directory should be named 'carddav'

It was not clear to me that we have to name the plugin dir 'carddav'.
Let's say you cloned rcmcarddav as is, it doesn't work because roundcube then looks for plugins/rcmcarddav/rcmcarddav.php.
Merge pull request #13 from jeekajoo/patch-1

precise that the plugin directory should be named 'carddav'