Add a FileDescriptor for notification of data available to read.

def addReader(self, selectable):
        """Add a FileDescriptor for notification of data available to read."""
        try:
            self._reads[selectable].resume()
        except KeyError:
            self._reads[selectable] = g = Stream.spawn(self, selectable, 'doRead')
            self.addToGreenletPool(g)