Add docs for common questions / use-cases

[crandmck]

Per discussion, we want to address the most common Express questions people ask on Stack Overflow (and perhaps elsewhere). Ideally, this will help to avoid people finding old, outdated info on S.O. and other places

I'd like to structure this better than just as a FAQ, but we'll need to see if we can find a better way to organize it.

Here are the working notes on this: https://github.com/strongloop/expressjs.com/wiki/Popular-Stackoverflow-questions .