Update migration-from-gson.md

Author: taowen

migration-from-gson.md

@@ -7,7 +7,7 @@ title: How to extend
 
 Jsoniter support gson annotation, migrating from gson to jsoniter should be easy.
 
-```
+```java
 public static class TestObject {
     @Expose(deserialize = false)
     public String field1;
@@ -21,9 +21,33 @@ TestObject obj = gson.fromJson("{\"field1\":\"hello\"}", TestObject.class);
 
 will be
 
-```
+```java
 GsonCompatibilityMode config = new GsonCompatibilityMode.Builder()
         .excludeFieldsWithoutExposeAnnotation()
         .build();
 TestObject obj = JsonIterator.deserialize(config, "{\"field1\":\"hello\"}", TestObject.class);
 ```
+
+serialization also works
+
+```java
+public static class TestObject {
+    public String field1 = "hello";
+}
+
+Gson gson = new GsonBuilder()
+        .setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE)
+        .create();
+String output = gson.toJson(new TestObject());
+assertEquals("{\"Field1\":\"hello\"}", output);
+```
+
+will be
+
+```java
+GsonCompatibilityMode config = new GsonCompatibilityMode.Builder()
+        .setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE)
+        .build();
+String output = JsonStream.serialize(config, new TestObject());
+assertEquals("{\"Field1\":\"hello\"}", output);
+```