Updated Readme to reflect arguments to script

Also added info about publishing directory structure.

README.md

@@ -2,6 +2,41 @@
 
 This Python script generates empty ESGF- and CMIP6-compliant files for offline radiative transfer calculations. The script uses the netCDF4 module. 
 
-The script requires the RFMIP-IRF input profiles from the [input4MIPs site](https://esgf-node.llnl.gov/search/input4mips/?institution_id=UColorado&target_mip_list=RFMIP).
+The script requires the RFMIP-IRF input profiles from the [input4MIPs site](https://esgf-node.llnl.gov/search/input4mips/?institution_id=UColorado&target_mip_list=RFMIP). Get help with the --help flag to the script. 
 
-Users will also want to edit the model-specific information starting on line 30 of the script, being careful to follow the CMIP6 Controlled Vocabulary entries for their model and institution. It would also be reasonable to update the "create_date" attribute when wrtiting data to the file, being careful to preserve the required formatting. 
+The script takes three optional arguments: 
+```
+  --source_id SOURCE_ID
+                        Source ID, must match CMIP Controlled Vocabulary at
+                        https://github.com/WCRP-
+                        CMIP/CMIP6_CVs/blob/master/CMIP6_source_id.json
+  --forcing_index FORCING_INDEX
+                        Forcing index (1 = all available greenhouse gases; 2 =
+                        CO2, CH4, N2O, CFC11eq; 3 = CO2, CH4, N2O, CFC12eq,
+                        HFC-134eq)
+  --physics_index PHYSICS_INDEX
+                        Physics index, e.g. for different approximations
+```
+If no arguments are provided the script produces files for `LBLRTM-12-8`. 
+
+It would  be reasonable to update the "create_date" attribute when writing data to the file, being careful to preserve the required formatting. 
+
+If you are publishing the data on the ESGF (rather than providing it to a publisher) recall that the directory structure needs to follow the conventions described in the [CMIP6 conventions document](https://goo.gl/v1drZl)
+
+Directory structure =
+<mip_era>/
+<activity_id>/
+<institution_id>/
+<source_id>/
+<experiment_id>/
+<member_id>/
+<table_id>/
+<variable_id>/
+<grid_label>/
+<version>
+
+Note:
+
+<version> has the form “vYYYYMMDD” (e.g., “v20160314”), indicating a representative date for the version.   Note that files contained in a single <version>  subdirectory at the end of the directory path should represent all the available time-samples reported from the simulation; a time-series can be split across several files, but all the files must be found in the same subdirectory.  This implies that <version> will not generally be the actual date that all files in the subdirectory were written or published.
+
+So an example directory will be:   CMIP6/RFMIP/AER/LBLRTM-12-8/rad-irf/r1i1p1f1/Efx/rlu/gn/v20190215/