`RDRSv2.1` needs a few warnings as the file/directory structure and containing data are counterintuitive

[kasra-keshavarz]

Reporte by: @wknoben

Since the time steps of each dataset file is a bit weird (starting from 12PM of the file date and ending in date+1 1PM), it makes sense to include these on its documentation under its repository.

Also, one more note is that December 31st, 2018 file (2018123112.nc) is non-existent, so this could be added to the docs as well.