Better docs to remember user to install ngResource

![Screen Shot 2013-03-05 at 12 01 39 AM](https://f.cloud.github.com/assets/469908/219748/678d691a-8541-11e2-8cd2-de5af95c8e1c.png)

After look the issue #1438 I just added the "Installation section" to give better docs for us. I'll also keep working in this issue.
Let me know what you think about that.

Cheers.