Revisions to the "git forward" session

I made a bunch of changes to the first git tutorial.

- PDF slides motivating the version control
- Pulled out part of the tutorial in the Readme.md file for the second day.
- Reorganized a bit of the tutorial in the Readme.md file, making some of the exercises more explicit, reordering some things, simplifying some of the explanations, and making the exercises more explicit.