Added initial version of logging.md file.

[mlanting]

This PR is intended to address https://github.com/ros2/design/issues/314 and facilitate discussion around what we want the user experience to look like.

This is a work in progress as I'm still trying to understand some areas of the launch system so I can more completely and accurately describe the status quo, but I want to get something pushed out for people to start commenting on and providing suggestions or corrections.