`weather-dl`: Document the -n argument

[Alex Merose]

In an oversight, a recent feature of weather-dl has not been documented. Let's make known to readers of the tool's README that the flag exists and describe what it does. Further, let's add a usage example.

[Himanshu]

Hi, i would like to work on this issue. Please guide me further on what needs to be done

[Alex Merose]

Sure thing. Please update the usage section of this doc: weather_dl/README.md

• install weather-dl and run --help on it
• copy the arg description of -n from there into the readme, following the existing format.
• add a usage example in the example section below