DOC: add example of using root_dir in condarc.

Motivation: I had a hard time getting started with ``conda init`` after ``pip install conda``. Init-ing in ``sys.prefix`` doesn't work on a normal Linux system due to permissions, and ``sudo conda init`` (besides not being desired) also doesn't work. Adding ``root_dir`` in condarc like ``envs_dir`` (i.e. a list) results in an incomprehensible error. So hopefully this helps someone.

A better solution would probably be to ask the user for a rootdir on ``conda init``.