add docs

docs/jsontype_seealso_cn.md

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+在fastjson中，可以使用Annotation JSONType#seeAlso配置，类似于JAXB中的XmlSeeAlso。用法如下：
+
+### JavaBean Config
+```java
+@JSONType(seeAlso={Dog.class, Cat.class})
+public static class Animal {
+}
+
+@JSONType(typeName = "dog")
+public static class Dog extends Animal {
+    public String dogName;
+}
+
+@JSONType(typeName = "cat")
+public static class Cat extends Animal {
+    public String catName;
+}
+```
+
+#### Usage
+```java
+Dog dog = new Dog();
+dog.dogName = "dog1001";
+
+String text = JSON.toJSONString(dog, SerializerFeature.WriteClassName);
+Assert.assertEquals("{\"@type\":\"dog\",\"dogName\":\"dog1001\"}", text);
+
+Dog dog2 = (Dog) JSON.parseObject(text, Animal.class);
+
+Assert.assertEquals(dog.dogName, dog2.dogName);
+```
+
+```java
+Cat cat = new Cat();
+cat.catName = "cat2001";
+
+String text = JSON.toJSONString(cat, SerializerFeature.WriteClassName);
+Assert.assertEquals("{\"@type\":\"cat\",\"catName\":\"cat2001\"}", text);
+
+Cat cat2 = (Cat) JSON.parseObject(text, Animal.class);
+
+Assert.assertEquals(cat.catName, cat2.catName);
+```