Recommend installing Command Line Tools via the full Xcode

I came across some issues trying to use the standalone Xcode Command Line Tools (as explained here), and Boxen now recommends using it through full Xcode, so this adds this recommendation to our README.

Does anyone know whether setting `sudo xcode-select --switch /path/to/xcode` is necessary when initially using full Xcode? I had to do that after installing it, but setting it to `/` during my previous adventures with the standalone version may have confused the situation...