README: Describe branching for contributors

[stevegt]

We used to have dev branch language for contributors in the README, but simplified it for the Nov 15th launch push. Need to dig that out of history, update, and restore.

[stevegt]

So I just now took a stroll through history in my own fork, and found this version by Mike in my local repo copy at 3d23b25 -- it was early days so not much as of that time, but this is what we originally had in regards to branching:

Contributing

1. Github supports all text editing directly on our github repository. Open any existing file and click the edit button as shown below...

You could also start a new file by opening an existing folder and clicking "Create new file" or "Upload files".

2. Make changes and then scroll down to the bottom to describe your changes (less than 80 characters in the title, and a more detailed description if requred past 80 chars). Click "Propose file change". This will create a forked repo on your account if it doesn't already exist, create a new branch called "patch-n" and offer to open the Pull Request for you.

3. Click the green "Create Pull Request" button.

4. A volunteer on the team will validate/merge the PR.

[stevegt]

Refer people to the right bits in https://help.github.com/categories/collaborating-with-issues-and-pull-requests/ for details on how to do this.