Add custom validation function doc

validation/README.md

@@ -63,7 +63,48 @@ Struct Tag Use:
 	}
 
 	func main() {
-		valid := Validation{}
+		valid := validation.Validation{}
+		u := user{Name: "test", Age: 40}
+		b, err := valid.Valid(u)
+		if err != nil {
+			// handle error
+		}
+		if !b {
+			// validation does not pass
+			// blabla...
+		}
+	}
+
+Use custom function:
+
+	import (
+		"github.com/astaxie/beego/validation"
+	)
+
+	type user struct {
+		Id   int
+		Name string `valid:"Required;IsMe"`
+		Age  int    `valid:"Required;Range(1, 140)"`
+	}
+
+	func IsMe(v *validation.Validation, obj interface{}, key string) {
+		name, ok:= obj.(string)
+		if !ok {
+			// wrong use case?
+			return
+		}
+
+		if name != "me" {
+			// valid false
+			v.SetError("Name", "is not me!")
+		}
+	}
+
+	func main() {
+		valid := validation.Validation{}
+		if err := validation.AddCustomFunc("IsMe", IsMe); err != nil {
+			// hadle error
+		}
 		u := user{Name: "test", Age: 40}
 		b, err := valid.Valid(u)
 		if err != nil {