'--user-dir' documentation (correct)

docs/overview.rst

@@ -46,3 +46,25 @@ decorators, for example::
 Once registered, new plug-ins can be used with the existing :ref:`Command-line
 Tools`. See the Tutorial sections for more detailed walkthroughs of how to add
 new plug-ins.
+
+**Loading plug-ins from another directory**
+
+New plug-ins can be defined in a custom module stored in the user system. In order to import the module, and make the plugin available to *fairseq*, the command line supports the ``--user-dir`` flag that can be used to specify a custom location for additional modules to load into *fairseq*.
+
+For example, assuming this directory tree::
+
+  /home/user/my-module/
+  └── __init__.py
+
+with ``__init__.py``::
+
+  from fairseq.models import register_model_architecture
+  from fairseq.models.transformer import transformer_vaswani_wmt_en_de_big
+
+  @register_model_architecture('transformer', 'my_transformer')
+  def transformer_mmt_big(args):
+      transformer_vaswani_wmt_en_de_big(args)
+
+it is possible to invoke the ``train.py`` script with the new architecture with::
+
+  python3 train.py ... --user-dir /home/user/my-module -a my_transformer --task translation