README: Update info with Clone

Signed-off-by: Patrick José Pereira <[email protected]>

README.md

@@ -97,7 +97,7 @@ There are cases when schema allows extensions for the certain type.
 In such cases we don't know in advance what fields must be present in Rust struct so we don't add them to output:
 
 ```rust
-#[derive(Default, PartialEq, Debug, YaSerialize, YaDeserialize)]
+#[derive(Default, Clone, PartialEq, Debug, YaSerialize, YaDeserialize)]
 #[yaserde(prefix = "tns", namespace = "tns: http://example.com")]
 pub struct MyType {
     #[yaserde(prefix = "tns", rename = "Parameters")]