Add doubly linked list to main README.

Author: trekhleb

README.md

@@ -24,6 +24,7 @@ the data.
 `B` - Beginner, `A` - Advanced
 
 * `B` [Linked List](src/data-structures/linked-list)
+* `B` [Doubly Linked List](src/data-structures/doubly-linked-list)
 * `B` [Queue](src/data-structures/queue)
 * `B` [Stack](src/data-structures/stack)
 * `B` [Hash Table](src/data-structures/hash-table)