Explain that schemas are for mandatory requirements

We do not host linting schemas here.

Author: handrews

CONTRIBUTING.md

@@ -259,6 +259,11 @@ As part of the publishing process, the YAML source files are converted to JSON,
 
 #### Schemas and OAS Requirements
 
+Not all uses of OAS Objects and fields are necessarily useful.
+However, the OAS does not forbid various questionable combinations, for
+reasons ranging from preserving possible future use with new features to
+maintaining compatibility with past versions that neglected to forbid something.
+
 The schemas published by the OpenAPI Initiative _only_ validate mandatory
 OAS requirements.  This means that a field value or combination with another
 field is only forbidden in a schema if there is a corresponding MUST / MUST NOT /