RequireJS Section Tweaking

I added a note to the build config bit about the fact that you have to use `.js` when specifying the output. When I first got started with RequireJS I always forgot it and it tripped me up so I figured a quick sentence about it would be nice.

Also got some queries:

L649: `var app_view = ...`. Any reason why it's `app_view` and not `appView` ?

In the build config section, is it worth talking about the `mainConfigFile` option? I use it a lot to avoid duplication across my config & my build files (https://github.com/jrburke/r.js/blob/master/build/example.build.js#L35).