modify engine type and readme md

Author: DeyiXu

README.md

@@ -1,2 +1,3 @@
-# outbox（实验性支持）
-发件箱模式
+# outbox 发件箱模式（实验性支持）
+
+基于[EventBus](https://github.com/nilorg/eventbus)，使用Go语言实现最终一致性的微服务分布式事务解决方案

gorm.go

@@ -14,9 +14,12 @@ import (
 	"gorm.io/gorm"
 )
 
+// EngineType engine type
+type EngineType int
+
 const (
-	// EngineTypeGorm ...
-	EngineTypeGorm = "gorm"
+	// EngineTypeGorm engine type for gorm
+	EngineTypeGorm EngineType = iota + 1
 )
 
 // gormEngine ...

outbox.go

@@ -92,7 +92,7 @@ type EngineOptions struct {
 }
 
 // New 创建
-func New(typ string, v interface{}, eventBus eventbus.EventBus, options ...*EngineOptions) (engine Engine, err error) {
+func New(typ EngineType, v interface{}, eventBus eventbus.EventBus, options ...*EngineOptions) (engine Engine, err error) {
 	if typ == EngineTypeGorm {
 		db := v.(*gorm.DB)
 		db.AutoMigrate(