add section on external standards

[kosack]

A description of what standards VODF follow (or will follow) for serialization.

• FITS standard (version 4):
  • Time headers
  • Observation headers
  • headers like REFERENC for linking to VODF.org
  • GTI extensions
  • ...

When writing down field names and header keys in the detailed model, we should also mark those that are from the FITS or other standards, so it's clear which ones we invented for VODF and which are standard