docs: add getHeadingList to docs

README.md

@@ -22,6 +22,40 @@ marked("# heading");
 // <h1 id="my-prefix-heading">heading</h1>
 ```
 
+## Get heading list
+
+`getHeadingList` is a function that is exported to provide the list of headings.
+
+The headings will each be an object with the following properties:
+ - `text`: The rendered HTML for the heading
+ - `level`: The heading level (1-7)
+ - `id`: The id given to the heading including any prefix
+
+```js
+import { marked } from "marked";
+import { gfmHeadingId, getHeadingList } from "marked-gfm-heading-id";
+
+marked.use(gfmHeadingId({prefix: "my-prefix-"}), {
+	hooks: {
+		postprocess(html) {
+			const headings = getHeadingList();
+
+			return `
+<ul id="table-of-contents">
+	${headings.map(({id, text, level}) => `<li><a href="#${id}" class="h${level}">${text}</a></li>`)}
+</ul>
+${html}`;
+		}
+	}
+});
+
+marked("# heading");
+// <ul id="table-of-contents">
+//   <li><a href="#my-prefix-heading" class="h1">heading</a></li>
+// </ul>
+// <h1 id="my-prefix-heading">heading</h1>
+```
+
 ## `options`
 
 | option |  type  | default | description                     |