'--user-dir' documentation (correct) (#447)

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