Update readme.md

Author: sushant097

coding_solutions/DataStructure_related/Tree/Readme.md

@@ -2,6 +2,7 @@
 | # | Problem | Solution |
 | :----: | :-----------------------------: | :----: |
 | 1 | [Maximum Depth of Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree/)| [python3](https://github.com/sushant097/Data-Structure-Algorithms-Collections-Python/blob/master/coding_solutions/DataStructure_related/Tree/MaxDepthTree.py) |
+| 2 | [Check Validity of BST](https://leetcode.com/problems/validate-binary-search-tree/)| [python3](https://github.com/sushant097/Data-Structure-Algorithms-Collections-Python/blob/master/coding_solutions/DataStructure_related/Tree/CheckValidityBST.py) |