updating DIRECTORY.md

Author: joydipb01

DIRECTORY.md

@@ -336,6 +336,7 @@
   * Morphological Operations
     * [Dilation Operation](digital_image_processing/morphological_operations/dilation_operation.py)
     * [Erosion Operation](digital_image_processing/morphological_operations/erosion_operation.py)
+    * [Skeletonization Operation](digital_image_processing/morphological_operations/skeletonization_operation.py)
   * Resize
     * [Resize](digital_image_processing/resize/resize.py)
   * Rotation