Added Parallelizable Subclass info to docs

Author: EltonCN

docs/concepts/parallel-task.rst

@@ -266,3 +266,10 @@ to set what should be determined to be collected at DAG construction time:
 
     # Then in the driver building pass in the configuration:
     .with_config(_config)
+
+Parallelizable Subclassing
+=============
+
+When annotating a function with `Parallelizable`, it is not possible to specify in the annotation what the type returned by the function will actually be, and these are not identified by a linter or other tools as static type checking. Especially for functions that can be used with or without Hamilton, this can be a problem.
+
+To solve this problem, it is possible to create subclasses of the `Parallelizable` classes. The ["Parallelizable Subclass" example](https://github.com/dagworks-inc/hamilton/blob/main/examples/parallelism/parallelizable_subclass) showcases how to do that.