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.

Built the codebase doc and saw the new text.