Clarify the branching / merging concepts

[stephwright]

Feedback suggested a visual representation.

[stephwright]

Options:
https://guides.github.com/activities/hello-world/branching.png
http://jlord.us/git-it/assets/imgs/branches.png
https://image.slidesharecdn.com/continuousintegrationingithub-160929224721/95/continuous-integration-in-github-6-638.jpg?cb=1475189404

[gallingerj]

I like the first image best! Clean, straightforward, and clarifying.