README.md

This model is converted from the original DKRR [repo](https://github.com/facebookresearch/FiD) and ported into Pyserini:

``` 
@misc{izacard2020distilling,
      title={Distilling Knowledge from Reader to Retriever for Question Answering},
      author={Gautier Izacard and Edouard Grave},
      year={2020},
      eprint={2012.04584},
      archivePrefix={arXiv},
      primaryClass={cs.CL}
}
```