docs(extensions): add extensions example

README.md

@@ -233,6 +233,39 @@ hexo.extend.filter.register('marked:tokenizer', function(tokenizer) {
 });
 ```
 
+#### Extensions
+
+It is also possible to customize the [extensions](https://marked.js.org/using_pro#extensions).
+For example, use [KaTeX](https://katex.org/) to render block-level math:
+
+```js
+const katex = require('katex');
+
+hexo.extend.filter.register('marked:extensions', function(extensions) {
+  // Info: `extensions` is an array.
+  extensions.push({
+    name: 'blockMath',
+    level: 'block',
+    tokenizer(src) {
+      const cap = /^\s{0,3}\$\$((?:[^\n]|\n[^\n])+?)\n{0,1}\$\$/.exec(src);
+
+      if (cap !== null) {
+        return {
+          type: 'blockMath',
+          raw: cap[0],
+          math: cap[1]
+        };
+      }
+
+      return undefined;
+    },
+    renderer(token) {
+      return `<p>${katex.renderToString(token.math, {displayMode: true})}</p>\n`;
+    }
+  });
+});
+```
+
 [Markdown]: https://daringfireball.net/projects/markdown/
 [marked]: https://github.com/chjj/marked
 [PHP Markdown Extra]: https://michelf.ca/projects/php-markdown/extra/#def-list