updating DIRECTORY.md

Author: Ramy-Badr-Ahmed

DIRECTORY.md

@@ -443,6 +443,7 @@
   * [Present Value](financial/present_value.py)
   * [Price Plus Tax](financial/price_plus_tax.py)
   * [Simple Moving Average](financial/simple_moving_average.py)
+  * [Straight Line Depreciation](financial/straight_line_depreciation.py)
   * [Time And Half Pay](financial/time_and_half_pay.py)
 
 ## Fractals