Update README for problem 136

Author: anirudhology

README.md

@@ -52,6 +52,7 @@ LeetCode solutions in Python
 | 130        | Surrounded Regions                                        | [Solution](problems/dfs/surrounded_regions.py)                                             | Medium     | DFS                               |
 | 131        | Palindrome Partitioning                                   | [Solution](problems/backtracking/palindrome_partitioning.py)                               | Medium     | Backtracking                      |
 | 133        | Clone Graph                                               | [Solution](problems/graph/clone_graph.py)                                                  | Medium     | Graph, DFS                        |
+| 136        | Single Number                                             | [Solution](problems/bit_manipulation/single_number.py)                                     | Easy       | Bit Manipulation                  |
 | 138        | Copy List With Random Pointer                             | [Solution](problems/linked_list/copy_list_with_random_pointer.py)                          | Medium     | Linked List, Hashing              |
 | 139        | Word Break                                                | [Solution](problems/dynamic_programming/word_break.py)                                     | Medium     | Dynamic Programming               |
 | 141        | Linked List Cycle                                         | [Solution](problems/linked_list/linked_list_cycle.py)                                      | Easy       | Linked List, Two Pointers         |