Update README.

Author: trekhleb

README.md

@@ -37,7 +37,7 @@ the data.
 * `B` [Queue](src/data-structures/queue)
 * `B` [Stack](src/data-structures/stack)
 * `B` [Hash Table](src/data-structures/hash-table)
-* `B` [Heap](src/data-structures/heap)
+* `B` [Heap](src/data-structures/heap) - max and min heap versions
 * `B` [Priority Queue](src/data-structures/priority-queue)
 * `A` [Trie](src/data-structures/trie)
 * `A` [Tree](src/data-structures/tree)