Add subtypes section in Java README.md document

Author: mikir

compiler/extensions/java/README.md

@@ -12,6 +12,19 @@ Zserio Java generator supports the Java SE 8 (LTS), the Java SE 11 (LTS) and the
 
 Although newer Java versions are not tested, they should work as well as long as they are backward compatible.
 
+## Subtypes
+
+Because Java language does not support aliases for types, Zserio subtypes in Java are resolved during
+API generation (resolved means that generated Java API uses always original type directly).
+
+However, there are some use cases where it would be beneficial to have a generated Java class for
+subtype types of compounds, e.g. to parse compound type which is given by a subtype class name.
+
+Therefore, for each subtype of compound, a Java class is generated which inherits the base type. These generated
+subtype classes are not used by generated code and are just meant to support applications.
+
+> Note: Subtypes of primitive types are not reflected in generated code at all.
+
 ## Compatibility check
 
 Java generator honors the `zserio_compatibility_version` specified in the schema. However note that only