Docs: example to split stats by entry point

The [inspectpack docs](https://github.com/FormidableLabs/inspectpack#note-multiple-entry-points) say that you can use this plugin to generate separate stats files per entrypoint. Since it seems like a relatively important use case to get the most out of inspectpack, can you add that as an example in this repo? I'm having some trouble understanding how to properly write a custom transform function to do that.

Regardless, I love the plugin and docs for this in general, so thanks!