update directory.md

Author: AdePhil

DIRECTORY.md

@@ -462,6 +462,7 @@
   * [Fractional Knapsack 2](greedy_methods/fractional_knapsack_2.py)
   * [Minimum Waiting Time](greedy_methods/minimum_waiting_time.py)
   * [Optimal Merge Pattern](greedy_methods/optimal_merge_pattern.py)
+  * [Gas Station](greedy_methods/gas_station.py)
 
 ## Hashes
   * [Adler32](hashes/adler32.py)