document thread safety and recommended singleton usage (#809)

doc.go

@@ -7,6 +7,14 @@ and has the ability to dive into arrays and maps of any type.
 
 see more examples https://github.com/go-playground/validator/tree/master/_examples
 
+Singleton
+
+Validator is designed to be thread-safe and used as a singleton instance.
+It caches information about your struct and validations,
+in essence only parsing your validation tags once per struct type.
+Using multiple instances neglects the benefit of caching.
+The not thread-safe functions are explicitly marked as such in the documentation.
+
 Validation Functions Return Type error
 
 Doing things this way is actually the way the standard library does, see the

validator_instance.go

@@ -89,6 +89,10 @@ type Validate struct {
 }
 
 // New returns a new instance of 'validate' with sane defaults.
+// Validate is designed to be thread-safe and used as a singleton instance.
+// It caches information about your struct and validations,
+// in essence only parsing your validation tags once per struct type.
+// Using multiple instances neglects the benefit of caching.
 func New() *Validate {
 
 	tc := new(tagCache)