Update README.md

README.md

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 # Dataset Card for OLM May 2022 Common Crawl
 
-Cleaned and deduplicated pretraining dataset, created with the OLM repo [here](https://github.com/huggingface/olm-datasets) from 15% of the May 2022 Common Crawl snapshot. 
+Cleaned and deduplicated pretraining dataset, created with the OLM repo [here](https://github.com/huggingface/olm-datasets) from 15% of the May 2022 Common Crawl snapshot.
+
+Note: `last_modified_timestamp` was parsed from whatever a website returned in it's `Last-Modified` header; there are likely a small number of outliers that are incorrect, so we recommend removing the outliers before doing statistics with `last_modified_timestamp`.