docs: Add introduction for ZeroValue and EpochValue methods

Author: kuafuRace

README.cn.md

@@ -1458,23 +1458,18 @@ person: {Date:2020-08-05 DateMilli:2020-08-05.999 DateMicro:2020-08-05.999999 Da
 ```
 
 ###### 自定义字段类型
+> 你可以实现 `carbon.DataTyper` 接口的 `DataType` 方法来重置数据库字段类型
 
 ```go
 type RFC3339Type string
 func (t RFC3339Type) Layout() string {
   return carbon.RFC3339Layout
 }
-func (t RFC3339Type) DataType() string {
-  return "datetime"
-}
 
 type ISO8601Type string
 func (t ISO8601Type) Format() string {
   return carbon.ISO8601Format
 }
-func (t ISO8601Type) DataType() string {
-  return "datetime"
-}
 
 type User struct {
   Customer1 *carbon.LayoutType[RFC3339Type] `json:"customer1"`

README.jp.md

@@ -1453,23 +1453,18 @@ person: {Date:2020-08-05 DateMilli:2020-08-05.999 DateMicro:2020-08-05.999999 Da
 ```
 
 ###### カスタムフィールドタイプ
+> データベースフィールドタイプをリセットするには、`carbon.DataTyper` インタフェースの `DataType` メソッドを実装できます
 
 ```go
 type RFC3339Type string
 func (t RFC3339Type) Layout() string {
   return carbon.RFC3339Layout
 }
-func (t RFC3339Type) DataType() string {
-  return "datetime"
-}
 
 type ISO8601Type string
 func (t ISO8601Type) Format() string {
   return carbon.ISO8601Format
 }
-func (t ISO8601Type) DataType() string {
-  return "datetime"
-}
 
 type User struct {
   Customer1 *carbon.LayoutType[RFC3339Type] `json:"customer1"`

README.md

@@ -1450,23 +1450,18 @@ person: {Date:2020-08-05 DateMilli:2020-08-05.999 DateMicro:2020-08-05.999999 Da
 ```
 
 ###### Customize type
+> You can implement the `DataType` method of the `carbon.DataTyper` interface to reset database field type
 
 ```go
 type RFC3339Type string
 func (t RFC3339Type) Layout() string {
   return carbon.RFC3339Layout
 }
-func (t RFC3339Type) DataType() string {
-  return "datetime"
-}
 
 type ISO8601Type string
 func (t ISO8601Type) Format() string {
   return carbon.ISO8601Format
 }
-func (t ISO8601Type) DataType() string {
-  return "datetime"
-}
 
 type User struct {
   Customer1 *carbon.LayoutType[RFC3339Type] `json:"customer1"`