$schema Guidance (#2266)

* chore: explain how $schema might work

* reordered and made it specifically only schema resources

* Update versions/3.1.0.md

Co-authored-by: Karen Etheridge <[email protected]>

* Update versions/3.1.0.md

Co-authored-by: Ben Hutton <[email protected]>

* new approach

Co-authored-by: Phil Sturgeon <[email protected]>
Co-authored-by: Karen Etheridge <[email protected]>
Co-authored-by: Ben Hutton <[email protected]>

Author: 4 people

versions/3.1.0.md

@@ -2336,6 +2336,14 @@ As such, inline schema definitions, which do not have a given id, *cannot* be us
 The [xml](#schemaXml) property allows extra definitions when translating the JSON definition to XML.
 The [XML Object](#xmlObject) contains additional information about the available options.
 
+###### Picking Schema Vocabularies
+
+It is important for tooling to be able to detect what meta-schema any given resource wishes to be processed with: JSON Schema Core, JSON Schema Validation, OpenAPI Schema Object, or some custom meta schema.
+
+`$schema` MAY be present in any Schema Object, and if present MUST be used to determine which dialect should be used when processing the schema.
+
+When `$schema` is not present, the default the following dialect MUST be assumed: `$schema: "https://spec.openapis.org/oas/3.1/schema-object"`.
+
 ##### Schema Object Examples
 
 ###### Primitive Sample