Update README for problem 66

Author: anirudhology

README.md

@@ -34,6 +34,7 @@ LeetCode solutions in Python
 | 56         | Merge Intervals                                           | [Solution](problems/intervals/merge_intervals.py)                                          | Medium     | Intervals, Array                      |
 | 57         | Insert Interval                                           | [Solution](problems/intervals/insert_interval.py)                                          | Medium     | Intervals, Array                      |
 | 62         | Unique Paths                                              | [Solution](problems/dynamic_programming/unique_paths.py)                                   | Medium     | Dynamic Programming                   |
+| 66         | Plus One                                                  | [Solution](problems/math/plus_one.py)                                                      | Easy       | Math                                  |
 | 70         | Climbing Stairs                                           | [Solution](problems/dynamic_programming/climbing_stairs.py)                                | Easy       | Dynamic Programming                   |
 | 73         | Set Matrix Zeroes                                         | [Solution](problems/matrix/set_matrix_zeroes.py)                                           | Medium     | Matrix                                |
 | 74         | Search A 2D Matrix                                        | [Solution](problems/binary_search/search_a_2d_matrix.py)                                   | Medium     | Binary Search                         |