Add README.

Author: trekhleb

src/data-structures/tree/avl-tree/README.md

@@ -0,0 +1,46 @@
+# AVL Tree
+
+In computer science, an AVL tree (named after inventors 
+Adelson-Velsky and Landis) is a self-balancing binary search 
+tree. It was the first such data structure to be invented. 
+In an AVL tree, the heights of the two child subtrees of any
+node differ by at most one; if at any time they differ by 
+more than one, rebalancing is done to restore this property.
+Lookup, insertion, and deletion all take `O(log n)` time in 
+both the average and worst cases, where n is the number of 
+nodes in the tree prior to the operation. Insertions and 
+deletions may require the tree to be rebalanced by one or 
+more tree rotations.
+
+Animation showing the insertion of several elements into an AVL 
+tree. It includes left, right, left-right and right-left rotations.
+
+![AVL Tree](https://upload.wikimedia.org/wikipedia/commons/f/fd/AVL_Tree_Example.gif)
+
+AVL tree with balance factors (green)
+
+![AVL Tree](https://upload.wikimedia.org/wikipedia/commons/a/ad/AVL-tree-wBalance_K.svg)
+
+### AVL Tree Rotations
+
+**Left-Left Rotation**
+
+![Left-Left Rotation](http://btechsmartclass.com/DS/images/LL%20Rotation.png)
+
+**Right-Right Rotation**
+
+![Right-Right Rotation](http://btechsmartclass.com/DS/images/RR%20Rotation.png)
+
+**Left-Right Rotation**
+
+![Left-Right Rotation](http://btechsmartclass.com/DS/images/LR%20Rotation.png)
+
+**Right-Left Rotation**
+
+![Right-Right Rotation](http://btechsmartclass.com/DS/images/RL%20Rotation.png)
+
+## References
+
+* [Wikipedia](https://en.wikipedia.org/wiki/AVL_tree)
+* [Tutorials Point](https://www.tutorialspoint.com/data_structures_algorithms/avl_tree_algorithm.htm)
+* [BTech](http://btechsmartclass.com/DS/U5_T2.html)