document --user option for core tool installs

[qwcode]

consider documenting (assuming we test it more) that you can get-pip.py --user, and mention --user as a possibility for the install of virtualenv (and wheel, and twine too, if not in a virtualenv)

see pypa/pip#1668

[adamn]

I believe this is complete (https://packaging.python.org/installing/#installing-to-the-user-site)

[theacodes]

Thanks, @adamn!