Add documentation

I've been working with people in the virtualdom room on IRC to create some documentation for people interested in working with virtual-dom directly. If you want to read them online, I've tagged the release at https://github.com/littleloops/virtual-dom-docs-wip/tree/0.1 

Although the docs cover things in both vtree and vdom, @neonstalwart recommended vtree as the place to keep the documentation. Let me know your thoughts on this, and if you find any problems in the docs.