Add list of extensions

Author: wooorm

readme.md

@@ -86,6 +86,13 @@ Array of mdast extensions (`Array.<MdastExtension>`, default: `[]`).
 
 [`Root`][root].
 
+## List of extensions
+
+*   [`syntax-tree/mdast-util-frontmatter`](https://github.com/syntax-tree/mdast-util-frontmatter)
+    — support frontmatter (YAML, TOML, etc)
+*   [`syntax-tree/mdast-util-gfm-strikethrough`](https://github.com/syntax-tree/mdast-util-gfm-strikethrough)
+    — support GFM strikethrough
+
 ## Security
 
 As Markdown is sometimes used for HTML, and improper use of HTML can open you up