add brief readme

README.md

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+# gz21-ocean-momentum pretrained models
+[gfdl-cm2.6-pangeo]: https://catalog.pangeo.io/browse/master/ocean/GFDL_CM2_6/
+[gz21-gh]: https://github.com/m2lines/gz21_ocean_momentum
+[gz21-data-hf]: https://huggingface.co/datasets/M2LInES/gfdl-cmip26-gz21-ocean-forcing
+
+[gz21-ocean-momentum][gz21-gh] models trained on the forcing data generated from
+the [GFDL CM2.6][gfdl-cm2.6-pangeo] dataset. (Some forcing is hosted on Hugging
+Face at [datasets/M2LInES/gfdl-cmip26-gz21-ocean-forcing][gz21-data-hf].
+
+See individual model directories for details (hyperparameters, code used).