Add documentation for ADC API requirements

[bcorrie]

Reflect the following as discussed at CRWG and Standards:

• TSV output SHOULD be supported for all data that is commonly represented in tabular structures
• All API endpoints SHOULD return JSON encodings
• For some API endpoints it is possible to request TSV files
• For any endpoints supporting TSV encodings these will be clearly documented in the API Spec and/or Documentation
• Endpoints that do not support TSV can reject TSV requests
• If API endpoint returns a field then the content of that field in JSON and TSV MUST be equivalent
• Bottom line: API Endpoints that are JSON or JSON+TSV should be defined in the spec/docs - the repositories need to follow this spec.

[bcorrie]

Need to clarify the wording for:

"TSV output SHOULD be supported for all data that is commonly represented in tabular structures"

This is more a file formats standards question, is that what we want to say?