Add AVL Tree.

Author: trekhleb

README.md

@@ -14,6 +14,7 @@
 7. [Trie](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/trie)
 8. [Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree)
     * [Binary Search Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree/binary-search-tree)
+    * [AVL Tree](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/tree/avl-tree)
 
 ## [Algorithms](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms)