Update README for problem 209

Author: anirudhology

README.md

@@ -88,6 +88,7 @@ LeetCode solutions in Go programming language.
 | 206        | Reverse Linked List                                       | [Solution](problems/linked_list/reverse_linked_list.go)                                    | Easy       | Linked List                           |
 | 207        | Course Schedule                                           | [Solution](problems/graph/course_schedule.go)                                              | Medium     | Graph, Topological Sort               |
 | 208        | Implement Trie (Prefix Tree)                              | [Solution](problems/trie/implement_trie.go)                                                | Medium     | Trie, Design                          |
+| 209        | Minimum Size Subarray Sum                                 | [Solution](problems/sliding_window/minimum_size_subarray_sum.go)                           | Medium     | Sliding Window                        |
 | 210        | Course Schedule II                                        | [Solution](problems/graph/course_schedule_ii.go)                                           | Medium     | Graph, Topological Sort               |
 | 211        | Design Add And Search Words Data Structure                | [Solution](problems/trie/design_add_and_search_words_data_structure.go)                    | Medium     | Trie, Design                          |
 | 212        | Word Search II                                            | [Solution](problems/trie/word_search_ii.go)                                                | Hard       | Trie, Backtracking                    |