Add unist-util-index to list of utilities

Author: wooorm

readme.md

@@ -144,6 +144,9 @@ A list of **VFile**-related utilities can be found at [**vfile**][vfile].
 *   [`unist-util-find-before`](https://github.com/wooorm/unist-util-find-before)
     — Find a node before another node;
 
+*   [`unist-util-index`](https://github.com/eush77/unist-util-index)
+    — Index nodes by property or computed key;
+
 *   [`unist-util-inspect`](https://github.com/wooorm/unist-util-inspect)
     — Node inspector;