updating DIRECTORY.md

Author: cclauss

DIRECTORY.md

@@ -179,6 +179,7 @@
   * [Shuffled Shift Cipher](ciphers/shuffled_shift_cipher.py)
   * [Simple Keyword Cypher](ciphers/simple_keyword_cypher.py)
   * [Simple Substitution Cipher](ciphers/simple_substitution_cipher.py)
+  * [Spiral Transposition](ciphers/spiral_transposition.py)
   * [Transposition Cipher](ciphers/transposition_cipher.py)
   * [Transposition Cipher Encrypt Decrypt File](ciphers/transposition_cipher_encrypt_decrypt_file.py)
   * [Trifid Cipher](ciphers/trifid_cipher.py)