docs: update readme

Author: VarusHsu

foo.xlsx

@@ -9,49 +9,51 @@ a easier use excel file create tool for golang
 ## Quick Start
 * define a struct with excel_header tag and implement `SheetName` method
 ```go
-type User struct {
-    Name     string    `excel_header:"姓名"`
-    Age      int       `excel_header:"年龄"`
-    Birthday time.Time `excel_header:"生日"`
-    Jobs     *string   `excel_header:"工作"`
+type Foo struct {
+    ID        int64      `excel_header:"id"`
+    Name      string     `excel_header:"name"`
+    CreatedAt time.Time  `excel_header:"created_at"`
+    DeletedAt *time.Time `excel_header:"deleted_at"`
 }
-func (u User) SheetName() string {
-    return "用户信息"
+func (u Foo) SheetName() string {
+    return "foo sheet name"
 }
 ```
 
 * construct some data
 ```go
-user1 := User{
-    Name: "张三",
-    Age: 18,
-    Birthday: time.Now(),
-    Jobs: nil,
+bar1DeletedAt := time.Date(2024, 1, 3, 15, 4, 5, 0, time.Local)
+sheetModels := []excelorm.SheetModel{
+    Foo{
+        ID:        1,
+        Name:      "Bar1",
+        CreatedAt: time.Date(2024, 1, 2, 15, 4, 5, 0, time.Local),
+        DeletedAt: &bar1DeletedAt,
+    },
+    Foo{
+        ID:        2,
+        Name:      "Bar2",
+        CreatedAt: time.Date(2024, 1, 2, 15, 4, 5, 0, time.Local),
+    },
 }
-user2 := User{
-    Name: "李四",
-    Age: 20,
-    Birthday: time.Now(),
-    Jobs: toPtr("程序员"),
-}
-sheetModels := make([]excelorm.SheetModel, 0)
-sheetModels = append(sheetModels, user1, user2)
 ```
 * write to excel file
 ```go
-err := excelorm.WriteExcelSaveAs("test.xlsx", sheetModels)
-if err != nil {
-    panic(err)
+if err := excelorm.WriteExcelSaveAs("foo.xlsx", sheetModels,
+    excelorm.WithTimeFormatLayout("2006/01/02 15:04:05"),
+    excelorm.WithIfNullValue("-"), 
+); err != nil {
+    log.Fatal(err)
 }
 ```
 * you can see the result in the file<br>
 
-| 姓名 | 年龄 | 生日                  | 工作  |
-|----|----|---------------------|-----|
-| 张三 | 18 | 2021-08-08 16:00:00 |     |
-| 李四 | 20 | 2021-08-08 16:00:00 | 程序员 |
+| id | name | created_at          | deleted_at          |
+|----|------|---------------------|---------------------|
+| 1  | Bar1 | 2024/01/02 15:04:05 | 2024/01/03 15:04:05 |
+| 2  | Bar2 | 2024/01/02 15:04:05 | -                   |
 
 
-[test.xlsx](test.xlsx)
+[foo.xlsx](foo.xlsx)
 
 * support multi-sheets by define more structs