Update docs to mention `included` addon hook

Sorry for the double PR, not sure if there is a cleaner way to do this. This is this docs update for #2381.

Includes a brief demonstration of using `included` to inject your own preprocessor into the app or parentAddon.
Yup, you are correct (I did not suggest that it was called for each build, just that it is called only when building). 

It is called once when the addon is to be included in the build process (but will never be called for commands that do not build).  This allows the addons to register preprocessors, add more `app.import`'s etc.