docs: add x-jsonschema-maxContains extension page

registries/_extension/x-jsonschema-maxContains.md

@@ -0,0 +1,43 @@
+---
+owner: baywet
+issue:
+description: The maximum number of array elements that may match contains, used when targeting OpenAPI versions that do not directly support maxContains.
+schema:
+  type: integer
+  minimum: 0
+objects: [ "Schema Object" ]
+layout: default
+---
+
+{% capture summary %}
+JSON Schema defines the `maxContains` keyword to set the maximum number of array elements that may match `contains`.
+
+The `x-jsonschema-maxContains` extension mirrors this JSON Schema keyword when targeting OpenAPI versions where the keyword is not directly available, serializing it as `x-jsonschema-maxContains`.
+
+It can appear as a property in the following objects: `{{page.objects|jsonify}}`.
+
+Used by: (informational)
+
+* [Microsoft.OpenApi](https://github.com/microsoft/OpenAPI.NET) (.NET OpenAPI library)
+{% endcapture %}
+
+{% capture example %}
+```yaml
+openapi: 3.0.4
+info:
+  title: My API
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    Roles:
+      type: array
+      items:
+        type: string
+      x-jsonschema-contains:
+        const: admin
+      x-jsonschema-maxContains: 1
+```
+{% endcapture %}
+
+{% include extension-entry.md summary=summary example=example %}