add chapter 4

chapter4/passfail/pass_fail.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"fmt"
+	"golang-book/head-first-go/chapter4/keyboard"
+	"log"
+)
+
+func main() {
+	fmt.Print("Enter a grade: ")
+	grade, err := keyboard.GetFloat()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var status string
+	if grade >= 60 {
+		status = "passing"
+	} else {
+		status = "failing"
+	}
+	fmt.Println("A grade of", grade, "is", status)
+}