feat: add PostgreSQL example to README (#279)

Author: hsluoyz

README.md

@@ -69,6 +69,46 @@ func main() {
 	e.SavePolicy()
 }
 ```
+
+## Simple PostgreSQL Example
+
+```go
+package main
+
+import (
+	"github.com/casbin/casbin/v2"
+	gormadapter "github.com/casbin/gorm-adapter/v3"
+	_ "github.com/lib/pq"
+)
+
+func main() {
+	// Initialize a Gorm adapter and use it in a Casbin enforcer:
+	// The adapter will use the PostgreSQL database named "casbin".
+	// If it doesn't exist, the adapter will create it automatically.
+	// You can also use an already existing gorm instance with gormadapter.NewAdapterByDB(gormInstance)
+	a, _ := gormadapter.NewAdapter("postgres", "host=127.0.0.1 port=5432 user=postgres password=postgres sslmode=disable") // Your driver and data source.
+	e, _ := casbin.NewEnforcer("examples/rbac_model.conf", a)
+	
+	// Or you can use an existing DB "abc" like this:
+	// The adapter will use the table named "casbin_rule".
+	// If it doesn't exist, the adapter will create it automatically.
+	// a := gormadapter.NewAdapter("postgres", "host=127.0.0.1 port=5432 user=postgres password=postgres dbname=abc sslmode=disable", true)
+
+	// Load the policy from DB.
+	e.LoadPolicy()
+	
+	// Check the permission.
+	e.Enforce("alice", "data1", "read")
+	
+	// Modify the policy.
+	// e.AddPolicy(...)
+	// e.RemovePolicy(...)
+	
+	// Save the policy back to DB.
+	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 ``TurnOffAutoMigrate(db *gorm.DB) *gorm.DB``. See example: 
 ```go