add chapter 4

chapter4/hi/main.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	"fmt"
+	"golang-book/head-first-go/chapter4/greeting"
+)
+
+func main() {
+	greeting.Hello()
+	greeting.Hi()
+
+	slc := []string{"hello", "world"}
+	fmt.Printf("%s %T %T", slc[0], slc, true)
+}