README: Added Python psutil to the list of dependencies.

Hello, Alan.

To help out other users, I thought it would be nice to add information about psutil to the README so they don't have to go track it down on their own like I did.  This pull request adds that information.

I have gotten Armory working on my Raspberry Pi, and I had to install "python-psutil" there.  On Arch Linux, I needed to install "python2-psutil" because they consider "python" to always mean the latest version of Python, and any package related to an older version must have the version number in its name.