Update the README with instructions about how to branch/fork and make pull requests.

[jwbowers]

We want people to easily contribute to and improve on the guides. So, we should update the README.md that people see when arriving at the repo with some basic instructions about forking and making pull requests (only people with access to the repo itself can use the branching model that I've been using for my own edits). Assigning this to @malisi to make a draft of this README.