Add example for schemars attributes

docs/examples/4-custom_settings.md

@@ -0,0 +1,13 @@
+---
+layout: default
+title: Custom Schema Settings
+parent: Examples
+nav_order: 4
+summary: Generating a schema using custom settings which changes how Option<T> is handled.
+---
+
+# Custom Schema Settings
+
+The `gen` module allows you to customise how schemas are generated. For example, the default behaviour for `Option<T>` is to include `null` in the schema's `type`s, but we can instead add a `nullable` property to its schema:
+
+{% include example.md name="custom_settings" %}