Add a section at 'jsx-in-depth' talking about namespaced components

Hi guys,

A feature that I like so mush in JSX and use in a lot of situations is the namespaced component that was created in version 0.11. So, I saw that have just a simple [line of text in a blog post](http://facebook.github.io/react/blog/2014/07/17/react-v0.11.html#jsx) talking about it. I think that is a good feature to have in the section [JSX in Depth](http://facebook.github.io/react/docs/jsx-in-depth.html) of the website.

Well, I created something short to explain that this feature can be used and avoid many boring things.

:smile: