docs(readme): add reference to validator.v8 docs, close #738 (#1024)

README.md

@@ -406,6 +406,8 @@ func main() {
 
 To bind a request body into a type, use model binding. We currently support binding of JSON, XML and standard form values (foo=bar&boo=baz).
 
+Gin uses [**go-playground/validator.v8**](https://github.com/go-playground/validator) for validation. Check the full docs on tags usage [here](http://godoc.org/gopkg.in/go-playground/validator.v8#hdr-Baked_In_Validators_and_Tags).
+
 Note that you need to set the corresponding binding tag on all fields you want to bind. For example, when binding from JSON, set `json:"fieldname"`.
 
 When using the Bind-method, Gin tries to infer the binder depending on the Content-Type header. If you are sure what you are binding, you can use BindWith.