Update README with general info about the dataset.

README.md

@@ -19,3 +19,15 @@ task_categories:
 task_ids:
 - multi-class-classification
 ---
+
+This is a [DocEE dataset](https://github.com/tongmeihan1995/docee) adapted for easy use with HuggingFace models.
+
+Dataset is described in [DocEE: A Large-Scale and Fine-grained Benchmark for Document-level Event Extraction](https://aclanthology.org/2022.naacl-main.291/) and can be downloaded from [here](https://github.com/tongmeihan1995/docee).
+
+Train split contains 21949 documents while test split contains 5536 documents. In total, dataset contains 27485 documents classified into 59 event types.
+
+## Differences from the original split(s)
+
+Originally, dataset is split into three splits: train, validation and test. For the purposes of this repository, original splits were joined back together and divided into train and test splits while making sure that splits were stratified across document sources (news and wiki) and event types.
+
+Finally, `date`, `title` and `metadata` columns were separated.