Update documentation with a section about QARTOD implementation

[leilabbb]

This is related to issue #355

Update the documentation with information about the QC process:

• GDAC implementation of QARTOD flags for real-time data (see QARTOD Variables - Users Info #314)
• Data providers' requirements to submit QC results to the DAC using a standardized QC variable structure and format.

[sarinamann-noaa]

Will discuss at GDAC meeting with large group