[Create Block] Adding documentation for the transformer property (#60445

)

* Adding documentation for the transformer property

* Remove unneeded line of code in example.

Co-authored-by: ryanwelcher <welcher@git.wordpress.org>
Co-authored-by: justintadlock <greenshady@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>

packages/create-block/docs/external-template.md

@@ -104,6 +104,32 @@ The following configurable variables are used with the template files. Template
 -   `style` (default: `'file:./style-index.css'`) – a frontend and editor style definition.
 -   `render` (no default) – a path to the PHP file used when rendering the block type on the server before presenting on the front end.
 -   `customBlockJSON` (no default) - allows definition of additional properties for the generated block.json file.
+-   `transformer` (default: `( view ) => view` ) - a function that receives all variables generated by the create-block tool and returns an object of values. This function provides the ability to modify existing values and add new variables.
+
+#### `transformer` examples
+
+This examples adds a generated value to the slug variable.
+
+```js
+transformer: ( view ) => {
+    const hex = getRandomHexCode();
+      return {
+         ...view,
+         slug: `${ view.slug }-${ hex }`,
+      };
+},
+```
+
+This example creates a new custom variable that can be used in the associated mustache templates as `{{customVariable}}`
+
+```js
+transformer: ( view ) => {
+      return {
+         ...view,
+         customVariable: `Custom Value`,
+      };
+},
+```
 
 ### `variants`