New article: Introduction to Shadow DOM

Post an introductory article along with [DevBytes video](https://www.youtube.com/watch?v=Is4FZxKGqqk).
All those posts can be found in the `Shadow DOM` tag page.

http://webcomponents.org/tags/shadow-dom/

I think it would be better to point people over there, since more and more content is going to be added here and they can find it in one single place.