update README

README.md

@@ -6,6 +6,8 @@ for performance numbers, see https://github.com/json-iterator/go-benchmark
 
 # DOM style api
 
+Jsoniter can work as drop in replacement for json.Unmarshal
+
 ```
 type StructOfTag struct {
 	field1 string `json:"field-1"`
@@ -32,6 +34,9 @@ func Test_reflect_struct_tag_field(t *testing.T) {
 
 # StAX style api
 
+When you need the maximum performance, the pull style api allows you to control every bit of parsing process. You
+can bind value to object without reflection, or you can calculate the sum of array on the fly without intermediate objects.
+
 Array
 
 ```
@@ -76,3 +81,57 @@ if iter.ReadString() != "b" {
     t.FailNow()
 }
 ```
+
+# Customization
+
+Of course, you can use the low level pull api to do anything you like. But most of the time,
+reflection based api is fast enough. How to control the parsing process when we are using the reflection api?
+json.Unmarshaller is not flexible enough. Jsoniter provides much better customizability.
+
+```
+func Test_customize_type_decoder(t *testing.T) {
+	RegisterTypeDecoder("time.Time", func(ptr unsafe.Pointer, iter *Iterator) {
+		t, err := time.ParseInLocation("2006-01-02 15:04:05", iter.ReadString(), time.UTC)
+		if err != nil {
+			iter.Error = err
+			return
+		}
+		*((*time.Time)(ptr)) = t
+	})
+	defer ClearDecoders()
+	val := time.Time{}
+	err := Unmarshal([]byte(`"2016-12-05 08:43:28"`), &val)
+	if err != nil {
+		t.Fatal(err)
+	}
+	year, month, day := val.Date()
+	if year != 2016 || month != 12 || day != 5 {
+		t.Fatal(val)
+	}
+}
+```
+
+there is no way to add json.Unmarshaller to time.Time as the type is not defined by you. Using jsoniter, we can.
+
+```
+type Tom struct {
+	field1 string
+}
+
+func Test_customize_field_decoder(t *testing.T) {
+	RegisterFieldDecoder("jsoniter.Tom", "field1", func(ptr unsafe.Pointer, iter *Iterator) {
+		*((*string)(ptr)) = strconv.Itoa(iter.ReadInt())
+	})
+	defer ClearDecoders()
+	tom := Tom{}
+	err := Unmarshal([]byte(`{"field1": 100}`), &tom)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+```
+
+It is very common the input json has certain fields massed up. We want string, but it is int, etc. The old way is to
+define a struct of exact type like the json. Then we convert from one struct to a new struct. It is just too much work.
+Using jsoniter you can tweak the field conversion.
+