Add a FAQ on the repo workflow to CONTRIBUTING.md

Hello folks,

I've been playing with scala repo for several months. Finding my way around was (and is) not an easy job. Watching @retronym, @xeno-by, @adriaanm, @paulp, @densh, @soc and others perform huge refactorings/changes is always amazing. How do they know where those things are? How do I even check that the thing I've just added does what I want it to? The way I gathered information presented in this PR was by asking scala-internals and bothering people via email. I can imagine, this might be quite annoying – answering same newbie questions over and over again. Do you think it makes sense to have some centralized place to keep these kinds of tips? Does it make sense to have it here, or should this be somewhere under http://docs.scala-lang.org? There's a lot of unwritten folklore around this repo, and writing it down would certainly help people find their way around, and attract new contributors. It would definitely help me.

I'm not sure about the format: my knowledge is very sparse here, so can be most easily presented as a FAQ. Just wanted to kick off the conversation and see if there are more people struggling with these kinds of things, or if I'm the only one having those problems.

Thanks.