Write the first two subsections.

* The hard sell.
Style improvements (manager -> account manager).
Add the section on repositories.
Fill out the Commits section.
Explain master and branches.

* Also swap the order of the last two sections, because
  pull requests generally happen before merges.
Add the final sections.

* With a little summary to touch on the core points.
Address issue comments.

* Remove some unnecessary parentheses.
* Trim the Repositories section down a bit.
Merge pull request #3 from rogerhutchings/i3-write-section-2

Write Section 2 - Version control overview