Docs: Link to AiiDA's docs for creating custom codes

docs/source/howto.rst

@@ -473,7 +473,8 @@ The ``command`` argument also accepts a pre-configured code instance directly:
     code = load_code('date@localhost')
     results, node = launch_shell_job(code)
 
-This approach can be used as an alternative to the previous example where the target computer is specified through the `metadata` argument.
+This approach can be used as an alternative to the previous example where the target computer is specified through the ``metadata`` argument.
+For more details on creating codes manually, please refer to `AiiDA's documentation <https://aiida.readthedocs.io/projects/aiida-core/en/latest/howto/run_codes.html#how-to-create-a-code>`_.
 
 
 Running with MPI