Add csharp solution to 268 Missing Number

Author: SPponmag

Diff for: csharp/268-Missing-Number.cs

@@ -0,0 +1,10 @@
+public class Solution {
+    public int MissingNumber(int[] nums) {
+        int sum = 0;
+        int total = nums.Length * (nums.Length + 1) / 2;
+        for (int i = 0; i < nums.Length; i++) {
+            sum += nums[i];
+        }
+        return total - sum;
+    }
+}