An addition to the docs

docs/index.md

@@ -35,8 +35,8 @@ $ pip install lark-parser
 * [Examples](https://github.com/lark-parser/lark/tree/master/examples)
 * Tutorials
     * [How to write a DSL](http://blog.erezsh.com/how-to-write-a-dsl-in-python-with-lark/) - Implements a toy LOGO-like language with an interpreter
-    * [How to write a JSON parser](json_tutorial.md)
-    * External
+    * [How to write a JSON parser](json_tutorial.md) - Teaches you how to use Lark
+    * Unofficial
         * [Program Synthesis is Possible](https://www.cs.cornell.edu/~asampson/blog/minisynth.html) - Creates a DSL for Z3
 * Guides
     * [How to use Lark](how_to_use.md)
@@ -44,6 +44,7 @@ $ pip install lark-parser
 * Reference
     * [Grammar](grammar.md)
     * [Tree Construction](tree_construction.md)
+    * [Visitors & Transformers](visitors.md)
     * [Classes](classes.md)
     * [Cheatsheet (PDF)](lark_cheatsheet.pdf)
 * Discussion