Add some basic information about the NWM operational configuration to the `nwm_client_new` package.

[jarq6c]

The nwm_client_new package assumes some basic familiarity with the National Water Model output. We could minimally link to the official landing page: https://water.noaa.gov/about/nwm and/or add an abbreviated description of the types of analysis and forecast data you can retrieve with this tool.

[jarq6c]

Could possibly link to these docs: https://github.com/microsoft/AIforEarthDataSets#noaa-national-water-model