Enable Xdebug autostart by default

It's usually annoying to remember that a specific GET or POST parameter is required when initiating a remote Xdebug session. Because we have Xdebug turned off by default, it doesn't seem to hurt to have autostart enabled when Xdebug is turned on.
Always enhancing the README...

May need to go back in and add some links to Wiki articles. I'm on a plane.

Still working on the best way to describe how to use VVV, but I think this comes a lot closer.