Extending Planet Foundation
You can either create a child theme for planet-foundation using the usual child theme process, or modify the theme itself. Start with planet-foundation/functions.php to begin learning the details.
API and File Structure
For a crude html documentation guide for the files and functions contained within the planet-foundation theme, go to planet-foundation/docs/api/index.html. This guide is a work in progress and I hope someday it will work and look much better.