Add types section

Author: wooorm

readme.md

@@ -18,6 +18,7 @@ The latest released version is [`4.0.0`][latest].
 
 *   [Introduction](#introduction)
     *   [Where this specification fits](#where-this-specification-fits)
+*   [Types](#types)
 *   [Nodes](#nodes)
     *   [`Parent`](#parent)
     *   [`Literal`](#literal)
@@ -90,6 +91,15 @@ languages.
 mdast relates to the [unified][] and [remark][] projects in that mdast syntax
 trees are used throughout their ecosystems.
 
+## Types
+
+If you are using TypeScript, you can use the unist types by installing them
+with npm:
+
+```sh
+npm install @types/mdast
+```
+
 ## Nodes
 
 ### `Parent`