Add description about retrieval process

README.md

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+This is a copy of the [Multi-News](https://huggingface.co/datasets/multi_news) dataset, with its input source documents replaced by a __sparse__ retriever. The retrieval pipeline used:
+
+- __query__: The `summary` field of each example
+- __corpus__: The union of all documents in the `train`, `validation` and `test` sets`
+- __retriever__: BM25 via [PyTerrier](https://pyterrier.readthedocs.io/en/latest/) with default settings _except_ for no stemming was applied.
+
+Retrieval results on the `test` set:
+
+|ndcg | recall@100 | recall@1000 | Rprec |
+| ----------- | ----------- | ----------- | ----------- |
+|0.8222 | 0.8242 | 0.8492 | 0.7233 |