Updated README

README.md

+# VOSpace Data Model
+
+## Generating beans from XML schema
+
+    cd xsd
+    xjc vospace.xsd
+
+### Changes to the online XSD files
+
+It seems that xjc does something wrong when retriving the imported XSD from the web, so the dependent files have been downloaded and relative path has been specified in `schemaLocation` attribute.
+
+In vospace.xsd:
+
+    <xsd:import namespace="http://www.ivoa.net/xml/UWS/v1.0" schemaLocation="./uws.xsd"/>
+
+In uws.xsd:
+
+    <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="./xlink.xsd"/>
+
+### Changes to the generated classes
+
+In package-info.java the following element has been added to serialize the XML keeping the namespace.
+
+    xmlns = {
+        @javax.xml.bind.annotation.XmlNs(
+                namespaceURI = "http://www.ivoa.net/xml/VOSpace/v2.0",
+                prefix = "vos"
+        )
+    }
+
+Some issues emerged in handling inheritance and namespaces in a way compatible both to JSON and XML formats. Moreover it was necessary to setup a workaround for filling the `xsi:type` of the root node:
+
+In Node.java type field and removeType() method have been added:
+
+    // Used for generating missing type attribute for root node. For child nodes it is filled automatically.
+    @XmlAttribute(name = "type", namespace = "http://www.w3.org/2001/XMLSchema-instance", required = false)
+    private String type;
+
+    /* This method exists to fix the issue with type attribute. See RemoveDuplicateTypeAdapter class. */
+    public void removeType() {
+        this.type = null;
+    }
+
+For JSON compatibility the following has been added to Node.java (annotation on class):
+
+    @JsonTypeInfo(use = JsonTypeInfo.Id.CUSTOM, property = "type", include = JsonTypeInfo.As.EXISTING_PROPERTY)
+    @JsonTypeIdResolver(NodeTypeJsonResolver.class)
+
+The `@JsonTypeInfo` tells to Jackson that the field type is used to handle inheritance. A custom type id resolver has been created to handle the `vos:` prefix.
+
+2 annotations have been added to each node subtype:
+
+    @XmlRootElement(name = "node")
+
+`@XmlRootElement` is necessary to parse single nodes. The value `"node"` has been specified because by default the bean would be serialized as `<unstructuredDataNode>`.