Update DIRECTORY.md to include biomedical statistical tests

- Add new Biomedical section in alphabetical order
- Include Mann Whitney U Test for independent group comparisons
- Include Wilcoxon Signed Rank Test for paired samples and one-sample tests
- Follow repository guidelines for categorizing new algorithms

Author: Brijesh03032001

DIRECTORY.md

@@ -2,6 +2,10 @@
 ## Association Algorithms
   * [Apriori](https://github.com/TheAlgorithms/R/blob/HEAD/association_algorithms/apriori.r)
 
+## Biomedical
+  * [Mann Whitney U Test](https://github.com/TheAlgorithms/R/blob/HEAD/Biomedical/mann_whitney_u_test.r)
+  * [Wilcoxon Signed Rank Test](https://github.com/TheAlgorithms/R/blob/HEAD/Biomedical/wilcoxon_signed_rank_test.r)
+
 ## Classification Algorithms
   * [Decision Tree](https://github.com/TheAlgorithms/R/blob/HEAD/classification_algorithms/decision_tree.r)
   * [Gradient Boosting Algorithms](https://github.com/TheAlgorithms/R/blob/HEAD/classification_algorithms/gradient_boosting_algorithms.r)
@@ -61,6 +65,7 @@
 ## Machine Learning
   * [Gradient Boosting](https://github.com/TheAlgorithms/R/blob/HEAD/machine_learning/gradient_boosting.r)
 
+
 ## Mathematics
   * [Amicable Numbers](https://github.com/TheAlgorithms/R/blob/HEAD/mathematics/amicable_numbers.r)
   * [Armstrong Number](https://github.com/TheAlgorithms/R/blob/HEAD/mathematics/armstrong_number.r)