Add a FileDescriptor for notification of data available to write.

def addWriter(self, selectable):
        """Add a FileDescriptor for notification of data available to write."""
        try:
            self._writes[selectable].resume()
        except KeyError:
            self._writes[selectable] = g = Stream.spawn(self, selectable, 'doWrite')
            self.addToGreenletPool(g)