ACCESS-ROM3 setup instructions

Awesome, thanks for doing this @mmr0

For the diag table, @Lizzie worked out that we could use an older version of the diag table for now but I suspect the long-term solution is to modify diag_table_source.yaml (the output_freq_units) and use this script to create our own diag table. I will have a fiddle around with this and share the results.