def addSubscription(subscriber, state, config):
    """
        Add new subscription to this node with given state.

        @param subscriber: JID of the new subscriptions' entity.
        @type subscriber: L{JID<twisted.words.protocols.jabber.jid.JID>}
        @param state: C{'subscribed'} or C{'pending'}
        @type state: C{str}
        @param config: Subscription configuration.
        @param config: C{dict}
        @return: deferred that fires on subscription.
        """
