Document convention for "predigested data" scripts

[tsjackson-noaa]

@yihungkuo pointed out that we ask POD developers to include scripts to generate their predigested obs data, but we don't have any convention on where they should put/organize them. It was suggested that we should standardize on a folder called obs_scripts inside the POD's code directory in diagnostics. Documentation needs to be updated to reflect this fact.