Add illustrations for Noms intro doc

[fxsjy]

I am trying to find documents of the core data structure in Noms. And, I found some docs in the "doc" directory, however, it has no graph illustration. Could you please provide some graph illustration of Nom's core data structrue. I think it will be better for others to understand how Noms works.

Thanks very much.

[aboodman]

The architectural overview is here:

https://github.com/attic-labs/noms/blob/master/doc/intro.md

I agree adding illustrations would be helpful. Patches welcome!