Add transform = ... attribute (#312)

This allows running arbitrary transforms on generated schemas when deriving `JsonSchema`

docs/1.1-attributes.md

@@ -49,6 +49,7 @@ TABLE OF CONTENTS
    - [`deprecated`](#deprecated)
    - [`crate`](#crate)
    - [`extend`](#extend)
+   - [`transform`](#transform)
    - [Doc Comments (`doc`)](#doc)
 
 </details>
@@ -326,10 +327,31 @@ Set on a container, variant or field to add properties (or replace existing prop
 The key must be a quoted string, and the value can be any expression that produces a type implementing `serde::Serialize`. The value can also be a JSON literal which can interpolate other values.
 
 ```plaintext
+#[derive(JsonSchema)]
 #[schemars(extend("simple" = "string value", "complex" = {"array": [1, 2, 3]}))]
 struct Struct;
 ```
 
+<h3 id="transform">
+
+`#[schemars(transform = some::transform)]`
+
+</h3>
+
+Set on a container, variant or field to run a `schemars::transform::Transform` against the generated schema. This can be specified multiple times to run multiple transforms.
+
+The `Transform` trait is implemented on functions with the signature `fn(&mut Schema) -> ()`, allowing you to do this:
+
+```rust
+fn my_transform(schema: &mut Schema) {
+   todo!()
+}
+
+#[derive(JsonSchema)]
+#[schemars(transform = my_transform)]
+struct Struct;
+```
+
 <h3 id="doc">
 
 Doc Comments (`#[doc = "..."]`)