Add list of related projects

Author: wooorm

readme.md

@@ -30,6 +30,7 @@ The latest released version is [`1.0.0`][latest].
 *   [Glossary](#glossary)
 *   [List of utilities](#list-of-utilities)
 *   [References](#references)
+*   [Related](#related)
 *   [Contribute](#contribute)
 *   [Acknowledgments](#acknowledgments)
 *   [License](#license)
@@ -414,6 +415,15 @@ See the [unist list of utilities][utilities] for more utilities.
     C. McCormack.
     W3C.
 
+## Related
+
+*   [hast](https://github.com/syntax-tree/hast)
+    — Hypertext Abstract Syntax Tree format
+*   [mdast](https://github.com/syntax-tree/mdast)
+    — Markdown Abstract Syntax Tree format
+*   [nlcst](https://github.com/syntax-tree/nlcst)
+    — Natural Language Concrete Syntax Tree format
+
 ## Contribute
 
 See [`contributing.md`][contributing] in [`syntax-tree/.github`][health] for