Update migration-from-gson.md

Author: taowen

migration-from-gson.md

@@ -4,3 +4,26 @@ title: How to extend
 ---
 
 # Migration from gson
+
+Jsoniter support gson annotation, migrating from gson to jsoniter should be easy.
+
+```
+public static class TestObject {
+    @Expose(deserialize = false)
+    public String field1;
+}
+
+Gson gson = new GsonBuilder()
+        .excludeFieldsWithoutExposeAnnotation()
+        .create();
+TestObject obj = gson.fromJson("{\"field1\":\"hello\"}", TestObject.class);
+```
+
+will be
+
+```
+GsonCompatibilityMode config = new GsonCompatibilityMode.Builder()
+        .excludeFieldsWithoutExposeAnnotation()
+        .build();
+TestObject obj = JsonIterator.deserialize(config, "{\"field1\":\"hello\"}", TestObject.class);
+```