Better documentation on how to make a contribution

[wtbarnes]

There are quite a few details about how to add content, but not about the process of setting up a development environment. We should probably have a quick walkthrough that would include:

1. Create a fork
2. Clone the fork
3. Make a change and test locally
4. Commit and push the change
5. Create PR
6. Review the preview on the PR
7. Merge if everything looks OK