Note on pip installation

Here is a note on installation of QuTip with [pip](http://www.pip-installer.org), that is, from https://pypi.python.org/pypi/qutip.

The good thing about pip is that, given other requirements are fulfilled (and people using Python for any kind of science usually already have the [SciPy](http://www.scipy.org/) stack), it is just one command, being system-independent. (Plus, it is a package manager, so it is easy to install, uninstall, upgrade, ...)

However, I am not sure, if the inclusion fits the style & structure of the installation instructions.