add chapter 5 and refactor root path

chapter5/average/main.go

@@ -0,0 +1,13 @@
+package main
+
+import "fmt"
+
+func main() {
+	numbers := [3]float64{71.8, 56.2, 89.5}
+	var sum float64 = 0
+	for _, num := range numbers {
+		sum += num
+	}
+
+	fmt.Println(sum)
+}