Change the "Getting Started" doc to recommend rebase over merge.

Using merges can be annoying and error-prone with a more-or-less centralized
development model, and I think this instruction has caused more than a couple
mistakes (the latest one being where one of our students merged his branch into
master, rather than the other way around).

This commit changes it to suggest rebase instead. While rebase can also be
annoying and error-prone, I think it's probably more likely to help people keep
a clean history.

Testing done: Built the codebase doc and saw the new text.

Reviewed at https://reviews.reviewboard.org/r/5399/