docs: add sqlite support and turn off AutoMigrate description (#161)

Signed-off-by: tangyang9464 <tangyang9464@163.com>

README.md

@@ -23,9 +23,9 @@ Based on [Officially Supported Databases](https://v1.gorm.io/docs/connecting_to_
 - PostgreSQL
 - SQL Server
 - Sqlite3
-> Since `` sqlite`` needs ``cgo`` support, it is only supported in branch ``sqlite`` instead of ``master``.See detail: [gorm-adapter#93](https://github.com/casbin/gorm-adapter/issues/93). If you want to use it, maybe you need to merge ``sqlite`` to ``master`` branch manually
+> gorm-adapter use ``github.com/glebarez/sqlite`` instead of gorm official sqlite driver ``gorm.io/driver/sqlite`` because the latter needs ``cgo`` support. But there is almost no difference between the two driver. If there is a difference in use, please submit an issue.
 
-You may find other 3rd-party supported DBs in Gorm website or other places.
+- other 3rd-party supported DBs in Gorm website or other places.
 
 ## Installation
 
@@ -69,7 +69,8 @@ func main() {
 	e.SavePolicy()
 }
 ```
-
+## Turn off AutoMigrate
+New an adapter will use ``AutoMigrate`` by default for create table, if you want to turn it off, please use API ``NewAdapterWithoutAutoMigrate(db *gorm.DB) (*Adapter, error)``. Find out more at [gorm-adapter#160](https://github.com/casbin/gorm-adapter/pull/160)
 ## Customize table columns example
 You can change the gorm struct tags, but the table structure must stay the same.
 ```go