Document folder layout in CONTRIBUTING.md

[gaearon]

It is currently not documented. We should do that to help contributions.
If you can help, please write here, and then send a PR!

[droidenator]

You're wanting to document the folders like node_modules, public, src and give some information about what they're used for? I'd be glad to give it a shot.

[gaearon]

Sorry, I meant the layout of this repository itself (hence CONTRIBUTING.md).
I think we touch on folder structure in the User Guide if you can make it better, please do!

[wdhorton]

I can work on this