Update README with statement of purpose

[vielmetti]

The README should include a statement of purpose, something roughly like this:

--

This repository is for demos of software and systems for the Works on Arm project.

Generally, we expect that these will allow you to start from a bare metal server on Packet and install and load and perhaps build a set of packages, do some useful work that proves that the packages behave correctly, and leave the system in a usable state from which someone could further explore.

The overall goal is to have these demos referenced from the Works On Arm website at http://worksonarm.com/explore and have a working demo of each package for which there's a claim that the software works.