Add example for schemars attributes

docs/examples/3-schemars_attrs.md

@@ -0,0 +1,13 @@
+---
+layout: default
+title: Using Schemars Attributes
+parent: Examples
+nav_order: 3
+summary: 'Deriving JsonSchema on types that use #[schemars] attributes to customise serialization behaviour.'
+---
+
+# Using Serde Attributes
+
+`#[serde(...)]` attributes can be overriden (or replaced) with `#[schemars(...)]` attributes, which behave identically. You may find this useful if you want to change the generated schema without affecting Serde's behaviour, or if you're just not using Serde.
+
+{% include example.md name="schemars_attrs" %}