add chapter 4

chapter4/const/planner/main.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"fmt"
+	"golang-book/head-first-go/chapter4/const/dates"
+)
+
+func main() {
+	days := 3
+	fmt.Println("Your appointment is", days, "days")
+	fmt.Println("with a follow-up in", days+dates.DaysInWeek, "days")
+}