Update README.md

Explain that this is a subset of the Tatoeba Challenge dataset.

README.md

@@ -27,3 +27,8 @@ configs:
   - split: test
     path: eng-spa/test-*
 ---
+
+This is a subset of the [Tatoeba Challenge dataset](https://github.com/Helsinki-NLP/Tatoeba-Challenge/). It contains only the English-to-Spanish data.
+
+The full Tatoeba Challenge dataset is available at [Helsinki-NLP/tatoeba_mt](https://huggingface.co/datasets/Helsinki-NLP/tatoeba_mt), but it does not work with the latest Datasets library because it contains custom code.
+A newer version which works with the latest Datasets library is available at [Helsinki-NLP/tatoeba_mt_train](https://huggingface.co/datasets/Helsinki-NLP/tatoeba_mt_train), however it does not currently contain all language pair (in particular, the Spanish-to-English subset that I needed is not there). [This issue](https://github.com/Helsinki-NLP/Tatoeba-Challenge/issues/42) may be fixed by the time you read this.