Add schema metadata

Added `id`, `$schema`, `description`

Author: webron

schemas/v3.0/schema.yaml

@@ -1,3 +1,6 @@
+id: https://spec.openapis.org/oas/3.0/schema/2019-04-02
+$schema: http://json-schema.org/draft-04/schema#
+description: Validation schema for OpenAPI Specification 3.0.X.
 type: object
 required:
   - openapi