Document Skenario Plugin Architecture

[josephburnett]

The HPA and VPA projects have adding a few layers to the architecture of Skenario. And the plugin architecture is not yet done. However, we should document what we have so far.

I recommend the following documents:

1. README.md (existing) -- answer the basic questions: a) what is Skenario / what does it look like, b) how can I run it, and c) where can I learn more.
2. docs/concepts.md (existing) -- just check for minor updates to the concepts (such as allowing removal of specific entities).
3. docs/architecture.md (new) -- a new document focusing on the layering of the software.

The architecture doc should either have a Plugins HOWTO section (how to write them) or we should have a separate doc. I'm leaning toward a subsection of architecture, because we haven't finished designing the plugin mechanism, so there might not be enough material to stand on it's own.