Add Contributor Guide

[robertlong]

We should add a guide for new contributors that explains our style guide, development setup, testing, and PR submission process.

[sbtron]

@pjcozzi @emackey Do you have any suggestions/starting points on how a contribution guide could be written? The Cesium ones are great could we start from there?

[pjcozzi]

By all means, please copy anything you want from the Cesium guides.