Add README.md

Author: dougiesquire

README.md

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+# GFDL generic tracers
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+This is a fork of the [NOAA-GFDL/ocean_BGC](https://github.com/NOAA-GFDL/ocean_BGC) repo managed by NOAA-GFDL. It contains a collection of tracers and associated code for use with the MOM and GOLD ocean models.
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+This fork includes generic tracer implementations of the suite of [WOMBAT models](https://www.access-nri.org.au/models/model-components/biogeochemistry-model-component-ocean-wombat/) used in [ACCESS earth-system models](https://www.access-nri.org.au/models/).