clarify GitHub tutorial

[julescubtree]

Ideas to improve:

1. Under "Make your file your own," we should further emphasize the part about committing directly to the current branch. (Additional strong text? A revised graphic?) This will help things match up under the "Submit a Pull Request" section.
2. Under "Submit a Pull Request," the first bullet could use additional direction, perhaps a hint or two (the repo name, and/or a nudge to look for the link above?).
3. Lastly, under "Submit a Pull Request," further emphasis in the fourth and especially bullets (maybe with graphics?) would help ensure tutorial followers are better able to match the tutorial.