Update README with basic information about DataWave GW work and detai…

…ls of the branch.

README.md

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 # CAM: The Community Atmosphere Model
 
+This fork of CAM is used for the development of machine-learnt gravity wave parameterisations
+developed as part of the DataWave project.
+
+The main working branch for this project is `datawave_ml` which was originally branched
+from the `cam6_3_139` tag.
+
 ## NOTE: This is **unsupported** development code and is subject to the [CESM developer's agreement](http://www.cgd.ucar.edu/cseg/development-code.html).
 
 ### CAM Documentation - https://ncar.github.io/CAM/doc/build/html/index.html