Added Series exercise to config.json

config.json

     "difference-of-squares",
     "largest-series-product",
     "queen-attack",
-    "minesweeper"
+    "minesweeper",
+    "series"
   ],
   "exercises": [
     {
       "slug": "minesweeper",
       "difficulty": 1,
       "topics": []
+    },
+    {
+      "slug": "series",
+      "difficulty": 1,
+      "topics": []
     }
   ],
   "deprecated": [