Added instructions for Apple contacts.app

#32 
There's a way to make the Contacts.app work with Radicale (on Mavericks). I could not use the Preferences of the Contacts.app because it froze everytime i tried to change the account. This guide applies the changes directly to the Configuration.plist.
I've updated the documentation page accordingly.

I believe this should work on every machine. But maybe someone should try this on another machine before pulling the commit into the official website.