Add custom validation example

_examples/custom-validation/main.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"fmt"
+
+	"gopkg.in/go-playground/validator.v9"
+)
+
+// MyStruct ..
+type MyStruct struct {
+	String string `validate:"is-awesome"`
+}
+
+// use a single instance of Validate, it caches struct info
+var validate *validator.Validate
+
+func main() {
+
+	validate = validator.New()
+	validate.RegisterValidation("is-awesome", ValidateMyVal)
+
+	s := MyStruct{String: "awesome"}
+
+	err := validate.Struct(s)
+	if err != nil {
+		fmt.Printf("Err(s):\n%+v\n", err)
+	}
+
+	s.String = "not awesome"
+	err = validate.Struct(s)
+	if err != nil {
+		fmt.Printf("Err(s):\n%+v\n", err)
+	}
+}
+
+// ValidateMyVal implements validator.Func
+func ValidateMyVal(fl validator.FieldLevel) bool {
+	return fl.Field().String() == "awesome"
+}