Validator documentation

[aradhakrishnanGFDL]

The validator needs some documentation for a user to try it out, and eventually consider folding it into the catalog building process. This issue only tracks the documentation aspects.

If I were a new user and I have catalogs csv, json available, how can I do a quick validation and understand what elements may be mission. (This is subjective of the application utilized, but the validator in the basic form needs to ensure the GFDL agreed upon required columns are not null.)