Add to RegisterCustomTypeFunc documentation

- Added additional notes to RegisterCustomTypeFunc to indicate that if a struct type is registered a url.Value must exist explicitly for the struct and not just the fields.

eg. url.Values{"User":"Name%3Djoeybloggs"} will call the custom type function with `User` as the type, however url.Values{"User.Name":"joeybloggs"} will not.

Fixes #14

Author: joeybloggs

README.md

@@ -1,7 +1,7 @@
 Package form
 ============
 <img align="right" src="https://raw.githubusercontent.com/go-playground/form/master/logo.jpg">
-![Project status](https://img.shields.io/badge/version-2.2.0-green.svg)
+![Project status](https://img.shields.io/badge/version-2.2.1-green.svg)
 [![Build Status](https://semaphoreci.com/api/v1/joeybloggs/form/branches/master/badge.svg)](https://semaphoreci.com/joeybloggs/form)
 [![Coverage Status](https://coveralls.io/repos/github/go-playground/form/badge.svg?branch=master)](https://coveralls.io/github/go-playground/form?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/go-playground/form)](https://goreportcard.com/report/github.com/go-playground/form)
@@ -217,6 +217,10 @@ decoder.RegisterCustomTypeFunc(func(vals []string) (interface{}, error) {
 		return time.Parse("2006-01-02", vals[0])
 	}, time.Time{})
 ```
+ADDITIONAL: if a struct type is registered, the function will only be called if a url.Value exists for
+the struct and not just the struct fields eg. url.Values{"User":"Name%3Djoeybloggs"} will call the
+custom type function with 'User' as the type, however url.Values{"User.Name":"joeybloggs"} will not.
+
 
 Encoder
 ```go

doc.go

@@ -210,6 +210,11 @@ Decoder
             return time.Parse("2006-01-02", vals[0])
         }, time.Time{})
 
+    ADDITIONAL: if a struct type is registered, the function will only be called
+    if a url.Value exists for the struct and not just the struct fields
+    eg. url.Values{"User":"Name%3Djoeybloggs"} will call the custom type function
+    with 'User' as the type, however url.Values{"User.Name":"joeybloggs"} will not.
+
 Encoder
 
     encoder.RegisterCustomTypeFunc(func(x interface{}) ([]string, error) {

form_decoder.go

@@ -112,8 +112,12 @@ func (d *Decoder) SetMaxArraySize(size uint) {
 	d.maxArraySize = int(size)
 }
 
-// RegisterCustomTypeFunc registers a CustomTypeFunc against a number of types
-// NOTE: this method is not thread-safe it is intended that these all be registered prior to any parsing
+// RegisterCustomTypeFunc registers a CustomTypeFunc against a number of types.
+// NOTE: This method is not thread-safe it is intended that these all be registered prior to any parsing
+//
+// ADDITIONAL: if a struct type is registered, the function will only be called if a url.Value exists for
+// the struct and not just the struct fields eg. url.Values{"User":"Name%3Djoeybloggs"} will call the
+// custom type function with `User` as the type, however url.Values{"User.Name":"joeybloggs"} will not.
 func (d *Decoder) RegisterCustomTypeFunc(fn DecodeCustomTypeFunc, types ...interface{}) {
 
 	if d.customTypeFuncs == nil {