Update README.md

Author: Andrew-Gammon

README.md

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 # MBCn
+
+Initial version of the codebase containing the code used to execute MBCn for the purposes of [NPCP](https://github.com/AusClimateService/npcp).
+
+Currently in the process of being adapted for more general ACS usage should the need arise.
+
+# Usage
+
+Running the MBCn requires 3 steps: 
+1) A chunking and initial correction step (npcp_chunk.py, run using the chunk_job.pbs job script)
+2) Main MBCn correction (npcp_calc.py, run using the calc_job.pbs job script)
+3) Conversion to NetCDF (npcp_convert.py, run using the convert_job.pbs job script)
+
+Steps 1 and 2 create intermediate Zarr files which should be stored in a temporary location such as a scratch directory.
+