Add extend attribute to docs

docs/1.1-attributes.md

@@ -48,6 +48,7 @@ TABLE OF CONTENTS
    - [`example`](#example)
    - [`deprecated`](#deprecated)
    - [`crate`](#crate)
+   - [`extend`](#extend)
    - [Doc Comments (`doc`)](#doc)
 
 </details>
@@ -314,6 +315,21 @@ struct Struct {
 }
 ```
 
+<h3 id="extend">
+
+`#[schemars(extend("key" = value))]`
+
+</h3>
+
+Set on a container, variant or field to add properties (or replace existing properties) in a generated schema. This can contain multiple key/value pairs and/or be specified multiple times, as long as each key is unique.
+
+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
+#[schemars(extend("simple" = "string value", "complex" = {"array": [1, 2, 3]}))]
+struct Struct;
+```
+
 <h3 id="doc">
 
 Doc Comments (`#[doc = "..."]`)