Add instruction how to use download_grib.sh

[Synkevych]

It must be described inside the file

[Synkevych]

For example: ./download_grib.sh 20201210 20201212
We also need to provide complete instruction for using Flexpart after weather is loaded.

[Synkevych]

Do not write information about the loaded weather to AVAILABLE if file has not been downloaded.