Improved docs to add bit about fetch and rebase

Due to the controversy over libuv, I ended up seeing their [CONTRIBUTING.md](https://github.com/joyent/libuv/blob/master/CONTRIBUTING.md) file, and I liked how they explained attaching the upstream as a remote and periodically using rebase. Overtime I want to improve our docs so that they are clear to users of all levels, but for now I'd like to get at least a few lines in there on the commands someone would run to do these things.