Update README.md

README.md

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-license: apache-2.0
+license: cc-by-4.0
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 This is the pre-training dataset for training the OlmoEarth pre-trained remote sensing foundation models.
 
-Documentation is on GitHub at https://github.com/allenai/olmoearth_pretrain/blob/main/docs/pretraining_dataset.md
+Documentation is on GitHub at https://github.com/allenai/olmoearth_pretrain/blob/main/docs/pretraining_dataset.md
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+The dataset is released under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/deed.en). It includes data from the following sources:
+
+- [Sentinel-2 L2A imagery](https://sentinels.copernicus.eu/sentinel-data-access/sentinel-products/sentinel-2-data-products/collection-1-level-2a) from the European Space Agency, available under the [Copernicus Sentinel Data and Service Legal Notice](https://sentinels.copernicus.eu/documents/247904/690755/Sentinel_Data_Legal_Notice)
+- [Sentinel-1 GRD IW vv+vh imagery](https://documentation.dataspace.copernicus.eu/APIs/SentinelHub/Data/S1GRD.html) from the European Space Agency, processed by Microsoft Planetary Computer, available under [CC BY 4.0](https://planetarycomputer.microsoft.com/dataset/sentinel-1-rtc)
+- [Landsat 8/9 OLI-TIRS imagery](https://www.usgs.gov/centers/eros/science/usgs-eros-archive-landsat-archives-landsat-8-oli-operational-land-imager-and) from NASA/USGS, [public domain](https://www.usgs.gov/centers/eros/data-citation?qt-science_support_page_related_con=0#qt-science_support_page_related_con)
+- The [USDA Cropland Data Layer](https://www.nass.usda.gov/Research_and_Science/Cropland/SARS1a.php) from USDA, public domain
+- [ERA5 monthly reanalysis data from ECMWF](https://cds.climate.copernicus.eu/datasets/reanalysis-era5-single-levels-monthly-means), available under [CC-BY](https://spdx.org/licenses/CC-BY-4.0)
+- [OpenStreetMap vector data](https://www.openstreetmap.org/), available under [ODbL](https://www.openstreetmap.org/copyright)