This line won't get into the README file. This is not the proper place to point to any other supplementary project, specially when it has no relation with ownCloud. We didn't test it, we don't know it and we have no reason to mention it instead of any other. 

Maybe the first sentence "For Calendar and Contacts sync, users can use any CalDAV and CardDAV clients they prefer."  could be valuable itself for ** regular users **, but I can't see why should it be into a file only visible for ** developers ** .