[game] Credited source for elo system

app/Models/Game.php

@@ -150,6 +150,7 @@ private function score_function($user, $alpha, $N) {
 
     private function calculate_elo($user_data)
     {
+        // Taken from https://towardsdatascience.com/developing-a-generalized-elo-rating-system-for-multiplayer-games-b9b495e87802
         $D = 400;
         $K = 32;
         $alpha = 2;