Update README.md

data/MULTIWOZ2.1/README.md

-Note that this is the mid 2019 version of MultiWOZ 2.1.
-The dataset has since been updated and the most recent version will therefore differ
+Note that this is a split (into train, val, test) of the mid 2019 version of MultiWOZ 2.1, which can be found at https://doi.org/10.17863/CAM.41572
+
+The dataset has since been updated and the most recent version of MultiWOZ 2.1 will therefore differ
 from the version we used for the experiments that we report in
 [TripPy: A Triple Copy Strategy for Value Independent Neural Dialog State Tracking](https://www.aclweb.org/anthology/2020.sigdial-1.4/)