Docs: Document store plugin

[Vheissu]

The Aurelia Store plugin has been ported from Aurelia 1 to Aurelia 2. We need to document the Aurelia Store plugin for v2, as well as consider adding in clearer documentation and examples.

One of the most confusing aspects of state management is knowing where it fits, when you should use it and how you should use it. I have extensive experience using the Store plugin and therefore, have answers for some of these questions already.