link to s.t.j.serialization namespace (#3701)

Author: tdykstra

xml/ns-System.Text.Json.xml

@@ -5,6 +5,8 @@
           <format type="text/markdown"><![CDATA[
 ## Remarks
 
+See also the <xref:System.Text.Json.Serialization> namespace, which contains attributes and APIs for advanced scenarios and customization specific to serialization and deserialization.
+
 For more information, see the [System.Text.Json overview](/dotnet/standard/serialization/system-text-json-overview).
           ]]></format>
       </remarks>