updating DIRECTORY.md

Author: cclauss

DIRECTORY.md

@@ -1284,6 +1284,8 @@
     * [Sol1](project_euler/problem_059/sol1.py)
   * Problem 060
     * [Sol1](project_euler/problem_060/sol1.py)
+  * Problem 061
+    * [Sol1](project_euler/problem_061/sol1.py)
   * Problem 062
     * [Sol1](project_euler/problem_062/sol1.py)
   * Problem 063