Add C# factorial algorithm (#344)

Author: nvillan16

Maths/Factorial/C#/Factorial.cs

@@ -0,0 +1,14 @@
+    class Factorial
+    {
+        public static long factorial(int inputNum)
+        {
+            if (inputNum == 1)
+            {
+                return 1;
+            }
+            else
+            {
+                return inputNum * factorial(inputNum - 1);
+            }
+        }
+    }