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

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.