Update README for problem 76

Author: anirudhology

README.md

@@ -18,6 +18,7 @@ LeetCode solutions in Python
 | 42         | Trapping Rain Water                            | [Solution](problems/array/trapping_rain_water.py)                                     | Hard       | Array, Two Pointers          |
 | 49         | Group Anagrams                                 | [Solution](problems/string/group_anagrams.py)                                         | Medium     | String, Hashing              |
 | 74         | Search A 2D Matrix                             | [Solution](problems/binary_search/search_a_2d_matrix.py)                              | Medium     | Binary Search                |
+| 76         | Minimum Window Substring                       | [Solution](problems/sliding_window/minimum_window_substring.py)                       | Hard       | Sliding Window               |
 | 84         | Largest Rectangle In Histogram                 | [Solution](problems/stack/largest_rectangle_in_histogram.py)                          | Hard       | Stack, Monotonic Stack       |
 | 121        | Best Time To Buy And Sell Stocks               | [Solution](problems/sliding_window/best_time_to_buy_and_sell_stocks.py)               | Easy       | Sliding Window               |
 | 125        | Valid Palindrome                               | [Solution](problems/string/valid_palindrome.py)                                       | Medium     | String, Two Pointers         |