Fixed #19437 - Clarified pip install instructions in contributing tutorial.

        pip install -e /path/to/your/local/clone/django/

    to link your cloned checkout into a virtual environment. This is a great

    option to isolate your development copy of Django from the rest of your

    system and avoids potential package conflicts.

    (where ``django`` is the directory of your clone that contains

    ``setup.py``) to link your cloned checkout into a virtual environment. This

    is a great option to isolate your development copy of Django from the rest

    of your system and avoids potential package conflicts.

__ http://www.virtualenv.org