feat: update CONTRIBUTING.md

Author: sailok.chinta

DIRECTORY.md

@@ -205,6 +205,7 @@
               * [PostOrderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/PostOrderTraversal.java)
               * [PreOrderTraversal](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/PreOrderTraversal.java)
               * [PrintTopViewofTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/PrintTopViewofTree.java)
+              * [QuadTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/QuadTree.java)
               * [RedBlackBST](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/RedBlackBST.java)
               * [SameTreesCheck](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/SameTreesCheck.java)
               * [SegmentTree](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/datastructures/trees/SegmentTree.java)