doc: Document what branch doc fixes go to

`CONTRIBUTING.md` should list the subsystems, to avoid [Jenkins barking](https://github.com/joyent/node/pull/5449#issuecomment-17745065) at [new committers](https://github.com/joyent/node/pull/5258#issuecomment-16083483). I haven't found an easily available or authoritative list of subsystems, so I pulled the ones from the last 100 commits on master. Hopefully there's a standard list somewhere, vs. just arbitrary strings followed by a semicolon, though I've seen both "doc" and "docs".