add brief discussion of sudo to install docs

As suggested by @demitri on astropy-dev, we may want to add something to the astropy install instructions that mentions using ``sudo python setup.py install`` instead of just ``python setup.py install``.  While many people know this is necessary, not everyone does.  So perhaps just a brief note the first time it comes up is in order (or possibly even for ``pip``, seeing as how sometimes it's needed there, too).