Add the Guide, add warning to tutorial.

In line with what @brson, @cmr, @nikomatsakis and I discussed this morning, my
redux of the tutorial will be implemented as the Guide. This way, I can work in
small iterations, rather than dropping a huge PR, which is hard to review.  In
addition, the community can observe my work as I'm doing it.

This adds a note in line with [this comment][reddit] that clarifies the state
of the tutorial, and the community's involvement with it.

[reddit]: http://www.reddit.com/r/rust/comments/28bew8/rusts_documentation_is_about_to_drastically/ci9c98k

Here's a screenshot of the warning: 

![warning](https://cloud.githubusercontent.com/assets/27786/3364870/1e3905fc-fb25-11e3-992b-9adcc41b4854.png)