Update README.md

Signed-off-by: Victor Smirnov <53015676+vityaman@users.noreply.github.com>

Author: vityaman

README.md

@@ -1,3 +1,11 @@
 # AVL Tree
 
 AVL Tree implementation in C++20.
+
+## Features
+
+- `AVL`, `Naive` binary search tree implementations
+- `OrderedSet` parametrized by `BST` type
+- `IndexedOrderedSet` with `O(logn)` kth element method
+- BST Node contains payload that is computed from its children
+- Common algorithms: `Search`, `Min/Max`, `Lower/Upper Bound`, `Successor/Predecessor`