Add git explanation

I noticed that if you start with the Installfest and then go straight to the Intro to Rails, you've set up git and everything talks about git, but it never really says what git _does_, beyond the fact that it's revision control. This gives a bit of context.

I thought it might really be helpful to add some context about history and branches, but thought the better of it, because that would quickly get overwhelming. But I do think it's important to give students a mental framework for what's going on when they commit code.