created variadic sum function

headFirstCodeMagnets/variadic.go

@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+func sum(numbers ...int) int {
+	var sum int = 0
+
+	for _, number := range numbers {
+		sum += number
+	}
+
+	return sum
+}
+
+func main() {
+	fmt.Println(sum(4, 1, 9, 2))
+	fmt.Println(sum(7))
+}